Bind various shortcut keyboard buttons to their respective actions

Refs: #44 (closed) & #1386 (closed)

Adds support for XF86Back and XF86Forward and such keys that some keyboards have.

I took the liberty of renaming the existing "THUNAR_WINDOW_ACTION_BACK_ALT" to "...ALT_1" in accordance to existing semantics ("THUNAR_WINDOW_ACTION_ZOOM_IN_ALT_1"), hope that's ok?