Move subsurface-core to core and qt-mobile to mobile-widgets

Having subsurface-core as a directory name really messes with
autocomplete and is obviously redundant. Simmilarly, qt-mobile caused an
autocomplete conflict and also was inconsistent with the desktop-widget
name for the directory containing the "other" UI.

And while cleaning up the resulting change in the path name for include
files, I decided to clean up those even more to make them consistent
overall.

This could have been handled in more commits, but since this requires a
make clean before the build, it seemed more sensible to do it all in one.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>

core/qt-gui.h

@@ -0,0 +1,15 @@
+#ifndef QT_GUI_H
+#define QT_GUI_H
+
+void init_qt_late();
+void init_ui();
+
+void run_ui();
+void exit_ui();
+
+#if defined(SUBSURFACE_MOBILE)
+#include <QQuickWindow>
+extern QObject *qqWindowObject;
+#endif
+
+#endif // QT_GUI_H