12ac248051
GTK IM operates in [coordinates relative to its client window](https://docs.gtk.org/gtk3/method.IMContext.set_cursor_location.html#:~:text=The%20location%20is%20relative%20to%20the%20client%C2%A0window.). Let GTK
IM use the native window that backs FlView, which is the same native
window that is used for key event processing - see GDK_KEY_XXX_MASK in
fl_view_realize().
This is an important step towards testable FlTextInputPlugin because it
eliminates the depency to GtkWidget's coordinate translation API and
that way paves the road for replacing FlTextInputPlugin's FlView
instance with a plain GdkWindow. In tests, that window can be an
instance of GdkOffscreenWindow.
There are no tests included yet but after a few more refactoring steps
it will be possible to execute tests like this: [fl_text_input_plugin_test.cc](f2e0d91b59/shell/platform/linux/fl_text_input_plugin_test.cc).
The other refactoring steps are visible at [jpnurmi/engine/commits/text-input-plugin](https://github.com/jpnurmi/engine/commits/text-input-plugin).