WorkspaceManager: Exclude a window that's about to be removed from count

[leolost2605]

Fixes #2145

Though probably not the same as #2145 (I've seen this one before the gesturepropertytransition got merged and the system version says stable) this probably also fixes #2159

We have to do this because a window that's about to be removed still appears in the list returned by Meta.Workspace.list_windows when window_removed is emitted (at least if it's just moved and not closed).

Also fixes an inconsistency, where when in the multitasking view, a workspace would be immediately removed when the last window on it was closed but not when it was moved to another one.

[lenemter]

Looked into mutter code and indeed it's the cause of the issue. _MetaWorkspace stores the windows but when list_windows () is called it fetches windows from the Meta.Display and filters them, so when window_removed is called workspace has removed the window from internal list, but not from the display's list. Should we report this upstream? Or maybe @tintou knows if this is intentional behaviour?