wayland: gtk_shell1 capability enumeration badly numbered

Submitted by James Henstridge @jamesh

Link to original bug (#787407)

Description

The gtk_shell1 Wayland protocol declares the following enumeration:

`<enum name="capability">`
  <entry name="global_app_menu" value="1"/>
  <entry name="global_menu_bar" value="2"/>
  <entry name="desktop_icons" value="3"/>
`</enum>`

These constants are then used to mask off bits in the value sent by the "capabilities" event in order to convert them to GdkScreen settings.

This is fine for the first two, but it means the "gtk-shell-shows-desktop" setting will only be set if global_app_menu and global_menu_bar are set. Presumably desktop_icons should be set to 4?

From the look of the mutter source code, it only ever sends the global_app_menu capability. So I guess this particular code path never gets tested.