Fix UI strings with plural forms

Otherwise those strings can not be extracted.

src/lib/app/browserwindow.cpp

@@ -1348,6 +1348,8 @@ void BrowserWindow::closeEvent(QCloseEvent* event)
 
     if (askOnClose && m_tabWidget->normalTabsCount() > 1) {
         CheckBoxDialog dialog(QMessageBox::Yes | QMessageBox::No, this);
+        //~ singular There is still %n open tab and your session won't be stored.\nAre you sure you want to close this window?
+        //~ plural There are still %n open tabs and your session won't be stored.\nAre you sure you want to close this window?
         dialog.setText(tr("There are still %n open tabs and your session won't be stored.\nAre you sure you want to close this window?", "", m_tabWidget->count()));
         dialog.setCheckBoxText(tr("Don't ask again"));
         dialog.setWindowTitle(tr("There are still open tabs"));

src/lib/downloads/downloaditem.cpp

@@ -185,12 +185,18 @@ QString DownloadItem::remaingTimeToString(QTime time)
         return tr("few seconds");
     }
     else if (time < QTime(0, 1)) {
+        //~ singular %n second
+        //~ plural %n seconds
         return tr("%n seconds", "", time.second());
     }
     else if (time < QTime(1, 0)) {
+        //~ singular %n minute
+        //~ plural %n minutes
         return tr("%n minutes", "", time.minute());
     }
     else {
+        //~ singular %n hour
+        //~ plural %n hours
         return tr("%n hours", "", time.hour());
     }
 }

src/lib/tabwidget/tabwidget.cpp

@@ -457,6 +457,8 @@ void TabWidget::closeTab(int index)
 
     if (m_menuTabs->isVisible()) {
         QAction* labelAction = m_menuTabs->actions().last();
+        //~ singular Currently you have %n opened tab
+        //~ plural Currently you have %n opened tabs
         labelAction->setText(tr("Currently you have %n opened tab(s)", "", count() - 1));
     }