Move Microsoft.Maui.Graphics.Gtk into the Maui repo

This was initially excluded from the merge in #8147 because it was not actually building in that repo.

Based on demand this can be moved here. However, in order to make sure that the code in the repo is actually buildable, we are not moving it right away.

Review the exclusion context in #8147 and inspect the Microsoft.Maui.Graphics.Gtk code and its current build setup. First establish what prevents it from building in the Maui repository; done means the code is moved and builds successfully there.

Written by the indexing model from the issue text.