gtkimcontext: Make gtk_im_context_activate_osk() public

Make this API public so that foreign "text" widgets (e.g. VteTerminal) have a chance to integrate this logic into their own event controllers, without having to craft the behavior of their own gestures around the built-in IM gesture.

In order to make it most useful for other backends, a GdkEvent argument and a boolean return value were added. This might be useful information for other platforms than Wayland, e.g. all investigation seems to hint that on Windows only the a11y keyboard is available programmatically via app launching, so the IM method implementation would need to set up its own policies for showing up the OSK (e.g. on touch events).