GtkTreeView: Rework the search window hack so it also works on Wayland

GtkTreeView: Rework the search window hack so it also works on Wayland

The search window of a tree view was implemented by showing without
making it visible by by positioning it outside the screen edge. This is
not possible on Wayland, so implement another method for being able to
enter text into a non-visible entry.

The new method is implemented by, before showing the window, pass the
key event directly to the IM context backing the entry. If the key
event triggered the context to commit new text or change the preedit
content, the search window is shown, and from that point the key events
are forwarded directly to the entry widget.

https://bugzilla.gnome.org/show_bug.cgi?id=756780