texthandle: Fix handle dragging on wayland

texthandle: Fix handle dragging on wayland

Text handles are subsurfaces on wayland, so sort of their own toplevel.
This made gtk_widget_translate_coordinates() to bail out there, resulting
in text handles being mispositioned and jumpy. To fix this, translate to
toplevel GtkWindow coordinates manually, and translate coordinates from
there.

Along the way, the coordinates reported in ::handle-dragged have been
fixed so there is no small jumps in either axis (most noticeable in the
X axis when you started dragging, and in the Y axis when moving between
lines of different heights.