Use XfwMonitor instead of GdkMonitor throughout

This changes the following from being/passing/returning GdkMonitor instances to returning XfwMonitor instances:

• XfwWindow:monitors
• xfw_window_get_monitors()
• XfwWorkspaceGroup:monitors
• xfw_workspace_group_get_monitors()
• XfwWorkspaceGroup::monitor-added
• XfwWorkspaceGroup::monitor-removed

Additionally, the GdkScreen used to be passed down into some of the class instances, and were exposed as properties. Now these are XfwScreen instances instead:

• XfwWorkspaceManager:screen
• XfwWorkspaceGroup:screen

For consistency, XfwScreen:screen has been renamed to XfwScreen:gdk-screen.

This also adds XfwScreen::monitor-added and XfwScreen::monitor-removed, which are similar to the same signals on GdkDisplay.

xfw_monitor_get_gdk_monitor() was added in !46 (merged), which can be used when a GdkMonitor instance is needed.

This also adds a XfwMonitor:workarea.