Add support to render the input panel on the client side for Gtk/Qt

[wengxt]

While with this we may lost feature of UI consistency (we could use the same rendering code as classic UI, so it is worth to give it a try), it is actually a good solution to workaround wayland positioning issue.

We need to be careful for:

1. non-standard Gtk/Qt application: chrome/firefox/libreoffice
2. Focus handling

[wengxt]

Also, we may need to be careful for Qt case, since we may only want to rely on QtGui instead of QWidget