Create gnome-pie-gdk-backends.vala

As Gnome-Pie will now run on the 'x11' GDK backend by default (in a Wayland session), we need to check whether that is available at runtime. Instead of only allowing the 'x11' GDK backend (Gdk.set_allowed_backends("x11");), dropping all wayland-specific workarounds and letting the app crash when the backend is not available, we opt to use a dedicated executable to check the availability of the 'x11' GDK backend at runtime. This needs a few extra shell calls via Posix.System("") - in daemon.vala - but is much cleaner and user-friendly this way.

IMPORTANT: due to the lack of CMake experience on my part, I might have placed this new file in the wrong location. It needs to be integrated into src/CMakeLists.txt, such that it is generated as a seperate executable (named gnome-pie-gdk-backends) but I don't know how to do that.

src/gnome-pie-gdk-backends.vala

@@ -0,0 +1,40 @@
+/////////////////////////////////////////////////////////////////////////
+// Copyright 2011-2018 Simon Schneegans
+//
+// Permission is hereby granted, free of charge, to any person obtaining
+// a copy of this software and associated documentation files (the
+// "Software"), to deal in the Software without restriction, including
+// without limitation the rights to use, copy, modify, merge, publish,
+// distribute, sublicense, and/or sell copies of the Software, and to
+// permit persons to whom the Software is furnished to do so, subject to
+// the following conditions:
+//
+// The above copyright notice and this permission notice shall be
+// included in all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+// EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+// MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
+// IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY
+// CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT,
+// TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE
+// SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+/////////////////////////////////////////////////////////////////////////
+
+// Basic executable to find available GDK backends at runtime.
+// Used internally, should never be visible for the user.
+
+public class GnomePieGdkBackends : Gtk.Application {
+	protected override void activate () {
+		var window = new Gtk.ApplicationWindow (this);
+		var label = new Gtk.Label ("Hello GDK Backends");
+		window.add (label);
+		window.set_title ("Welcome to Gnome-Pie!");
+		window.set_default_size (200, 100);
+		window.show_all ();
+	}
+}
+
+public int main (string[] args) {
+	return new GnomePieGdkBackends ().run (args);
+}