Set associated window for tray icon after showing main window

Currently we set the associated window by passing GUI as parent window to the
KStatusNotifierItem constructor. This will obtain the windowHandle for it and
save that.

Later when creating the WebView for the search plugin Qt deletes and recreates the
windowHandle for the main window, invalidating the windowHandle KStatusNotifierItem
has saved.

This causes showing/hiding via the tray icon to not work.

To avoid this manually set the associated window later when the handle won't change
any more

BUG: 483899
(cherry picked from commit cb058cff)