wayland: Separate touch pointer emulation into its own master pointer

wayland: Separate touch pointer emulation into its own master pointer

The common GDK code accounts for "pointer emulating" touch sequences to be
synchronized with the pointer position by the windowing system.

However on Wayland pointer and touch are completely independent, the backend
attempts to implement pointer emulation, but doesn't account for the
possible crossing events happening when the user switches from pointer to
touch or the opposite.

In order to fix this, and to ensure we don't have to interact with the
master pointer (which backs the wl_pointer), separate the touch interface
to have its own master pointer, and ensure crossing events are emitted on
it, so the picture of an "emulated pointer" is complete above the backend.

Inspired in a former patch by Jonny Lamb <jonnylamb@gnome.org>

https://bugzilla.gnome.org/show_bug.cgi?id=750845