Merge remote-tracking branch 'mono/master' into sevoku/gtk3

Conflicts:
	Xwt.Gtk/Xwt.Gtk.csproj
	Xwt.Gtk/Xwt.GtkBackend/GtkInterop.cs
	Xwt.Gtk/Xwt.GtkBackend/GtkWorkarounds.cs
[GTK] add Gtk3 PopoverBackend support
[GTK] let parent window hide Popover.
Popover should be only hidden, if the parent window gets
the focus. Gtk.Window doesn't know which window receives
the focus and hides when child windows (e.g. ComboBox)
steal the focus, too.
Merge remote-tracking branch 'upstream/master' into gtk3

Conflicts:
	Xwt.Gtk/Xwt.GtkBackend/Conversion.cs
Merge remote-tracking branch 'origin/fix-gtk-popover-backend'
Merge branch 'gtk3'

Conflicts:
	Xwt.Gtk/Xwt.Gtk.csproj
	Xwt.Gtk/Xwt.Gtk3.csproj
	Xwt.Gtk/Xwt.GtkBackend/GtkInterop.cs
	Xwt.Gtk/Xwt.GtkBackend/PopoverBackend.cs