QML: Add windows tutorial

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

qml/articles/9.Falkon_windows.md

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+# Falkon QML Tutorial - 9. Falkon windows
+
+Hello, in this chapter I will show you how to get work with windows.
+
+## Access windows api
+The windows API can be accessed through code.
+```qml
+Falkon.Windows
+```
+More information can be found in the
+[documentation](http://falkon.sgorava.xyz/docs/class_qml_windows.html).
+
+### Get windows
+To get to the windows we can use three methods `get(windowId)`,
+`getCurrent()` and `getAll()`. These methods return a window or array of
+windows of type `Window` (see
+[documentation](http://falkon.sgorava.xyz/docs/class_qml_window.html)
+) which van be manipulated further.
+
+Example:
+```qml
+/* Get a specific window with known ID */
+var window = Falkon.Windows.get(0)
+
+/* Get current window */
+var currentWindow = Falkon.Windows.getCurrent()
+
+/* Get all windows */
+var allWindows = Falkon.Windows.getAll()
+```
+
+### Create a new window
+To create a new window we have to supply it with URL and window type.  
+The `type` is optional and by default the `NewWindow` will be used.
+
+```qml
+Falkon.Windows.create({
+    url: 'https://falkon.org/'
+    type: Falkon.Enums.WindowType.NewWindow
+})
+```
+
+### Remove an existing window
+To remove an existing window we need to first know its ID in current
+session and than we can use the `remove` method.
+
+```qml
+Falkon.Windows.remove(WINDOW_ID)
+```
+
+### Available signals
+#### created(Window)
+Triggered when new window was created with the new window in parameter.
+
+#### removed(Window)
+Triggered when window was removed (closed) with the window as a
+parameter.
+
+## Window properties
+Each window has some properties which can be read, we can also get to
+all tabs which belong to that window.
+
+```qml
+window.id
+window.incognito
+window.title
+window.state
+window.type
+window.tabs
+window.focussed
+window.height
+window.width
+```
+
+More detailed information can be found in the
+[documentation](http://falkon.sgorava.xyz/docs/class_qml_window.html).
+
+## Example
+The example will create a sidebar in which will be a list of currently
+open Falkon windows with information and few buttons.
+
+![Windows list in the sidebar](../images/tutorial9/sidebar_windows_list.png)
+
+### Code
+The code for this example can be found at
+[extensions/qml_tutorial_9](../extensions/qml_tutorial_9)