Drop zenity dependency

As per the discussion in #1762 (closed), it seems like the ideal way forward is to drop 2 out of 3 use cases of zenity in mutter. Which are used to display a

• Display configuration confirmation

• Force quit a window dialog

These two are default implementations provided by Mutter and at least GNOME Shell & Gala (by elementary) do provide their own implementations, so I have dropped the implementations here for now.

• X11 session windows list

Marked as a draft until we reach an agreement on how to handle this one. I have replaced it with clutter actors. Ended up dropping it per Ray Strode's explantation regarding the usage of this dialog

Fixes #1762 (closed)