gui: mainwindow: Add CTRL+F and CTRL+W shortcuts

Add shortcuts to make entering queries more efficient

CTRL+F: Highlights rightmost filter, e. g. c:(word1 word2) would
highlight everything between (), so "word1 word" here. Alternatively,
highlights lone words, so p:(docs) word1, would highlight word1.

CTRL+W: Removes last filter or words.

gui/mainwindow.h

@@ -63,7 +63,8 @@ class MainWindow : public QMainWindow
 	unsigned int currentPreviewGeneration = 1;
 	void initSettingsTabs();
 	int currentSelectedScale();
-  private slots:
+	void processShortcut(int key);
+private slots:
 	void lineEditReturnPressed();
 	void treeSearchItemActivated(QTreeWidgetItem *item, int i);
 	void showSearchResultsContextMenu(const QPoint &point);