Add Tutorial2 - Basic Extension for QML

Signed-off-by: Juraj Oravec <jurajoravec@mailo.com>

qml/articles/2.Basic_extension.md

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+# Falkon QML Tutorial - 2. Basic extension
+
+This will create a basic extension which Falkon will be able to load and
+unload. It will also print text to the standard output and to see it
+start falkon from terminal.
+
+## File structure
+
+Each extension has to contain:
+
+ * metadata.desktop
+ * main.qml
+
+Anything else depend on the creator.
+
+## metadata.desktop
+This file contain information which are displayed in Falkon settings and
+some configuration.
+
+### Example file
+
+```
+[Desktop Entry]
+Name=Example QML
+Comment=Example QML extension
+Icon=
+Type=Service
+X-Falkon-Type=Extension/Qml
+
+X-Falkon-Author=Juraj Oravec
+X-Falkon-Email=jurajoravec@mailo.com
+X-Falkon-Version=1.0.0
+X-Falkon-Settings=false
+```
+
+#### Name
+Extension name shown in Falkon settings.
+
+#### Comment
+Extension description shown in Falkon settings.
+
+#### Icon
+Extension Icon shown in Falkon settings.  
+Path to icon file or icon name from icon theme.  
+For the final product it is best to provide the icon with the extension
+so that machines without this icon or on some obscure systems the icon
+will be present.
+
+#### Type
+Keep: `Service`  
+I did not find a use of this field in code yet (maybe legacy ?).
+
+#### X-Falkon-Type
+Falkon Extension type.  
+For QML extension use: `Extension/Qml`
+
+#### X-Falkon-Author
+Author`s name shown in Falkon settings.
+
+#### X-Falkon-Email
+Author`s email shown in Falkon settings.
+
+#### X-Falkon-Version
+Extension version shown in Falkon settings.
+
+#### X-Falkon-Settings
+If extension has a setting dialog.  
+Extension `Settings` button in Falkon preferences depends on this
+variable.  
+Possible values: `true` or `false`
+
+
+
+## main.qml
+Entry point to our program.  
+This file will be called when extension is loaded.  
+Some people consider leaving this file almost empty with only license
+and import line.  
+I will leave the decision up to you.
+
+### Example extension
+
+```qml
+import org.kde.falkon 1.0 as Falkon
+
+Falkon.PluginInterface {
+
+    init: function(state, settingsPath){
+        console.log(i18n('"Tutorial2" plugin loaded'))
+    }
+
+    testPlugin: function() {
+        return true
+    }
+
+    unload: function() {
+        console.log(i18n('"Tutorial2" plugin unloaded'))
+    }
+
+}
+```
+
+The plugin / extension inherits from `Falkon.PluginInterface`[1].
+The full code can be found at
+[extensions/qml_tutorial_2](../extensions/qml_tutorial_2)
+
+### Methods used
+
+#### init()
+Called when extension is loaded.
+
+* **state**: possible values
+   * **Falkon.PluginInterface.InitState.StartupInitState**  
+Normal init state during Falkon start, nothing to do for us.
+   * **Falkon.PluginInterface.LateInitState**  
+The extension was loaded long after Falkon started.  
+In this case we need to manually call functions which would normally
+be called during the start.  
+The exact usecase will be shown in one of the next chapters.
+* **settingsPath**: Contains path to an extension setting directory
+
+#### unload()
+Called when the plugin is uloading by being disabled in settings or when
+Falkon is closing.
+
+#### testPlugin()
+Method to check if plugin is compatible with current Falkon version.  
+Return `true` if is and `false` if not.
+
+
+
+[1]: https://github.com/KDE/falkon/blob/master/src/lib/plugins/qml/qmlplugininterface.h

qml/extensions/qml_tutorial_2/main.qml

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+import org.kde.falkon 1.0 as Falkon
+
+Falkon.PluginInterface {
+
+    init: function(state, settingsPath){
+        console.log(i18n('"Tutorial2" plugin loaded'))
+    }
+
+    testPlugin: function() {
+        return true
+    }
+
+    unload: function() {
+        console.log(i18n('"Tutorial2" plugin unloaded'))
+    }
+
+}

qml/extensions/qml_tutorial_2/metadata.desktop

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+[Desktop Entry]
+Name=Tutorial2 QML
+Comment=Example QML extension
+Icon=
+Type=Service
+X-Falkon-Type=Extension/Qml
+
+X-Falkon-Author=Juraj Oravec
+X-Falkon-Email=jurajoravec@mailo.com
+X-Falkon-Version=1.0.0
+X-Falkon-Settings=false