Library: add a shortcut to focus search line

The Ctrl+F shortcut gives focus to a search bar in many applications.
In this case it allows to search the library without touching a mouse.

YACReaderMacOSXSearchLineEdit::setFocus() will have to be implemented to
make the shortcut work on macOS.

YACReaderLibrary/library_window.h

@@ -213,6 +213,7 @@ public:
     QAction *forceCoverExtractedAction;
     QAction *deleteComicsAction;
 
+    QAction *focusSearchLineAction;
     QAction *showEditShortcutsAction;
 
     QAction *updateFolderAction;