Fix `wl_pointer` enter events not consistently emitted when mapping a window on Wayland

Merged Olivier Fourdan requested to merge ofourdan/mutter:wayland-enter into master

This is to fix issue #1016 (closed)

As it appears, there are multiple causes to the problem reported:

  1. The clutter actor underneath the pointer might not be up-to-date
  2. various animation effects may defeat picking if done too early

This MR aims at fixing the problem while cleaning some very old code from 2013 that was added in meta_window_show() to fix the issue in 2013 (commit 3f022ca9)

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