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>

mobile-widgets/qml/icons/context-menu.svg

@@ -0,0 +1 @@
+<svg xmlns="http://www.w3.org/2000/svg" width="48" height="48" viewBox="0 0 48 48"><path d="M24 16c2.21 0 4-1.79 4-4s-1.79-4-4-4-4 1.79-4 4 1.79 4 4 4zm0 4c-2.21 0-4 1.79-4 4s1.79 4 4 4 4-1.79 4-4-1.79-4-4-4zm0 12c-2.21 0-4 1.79-4 4s1.79 4 4 4 4-1.79 4-4-1.79-4-4-4z"/></svg>

mobile-widgets/qml/icons/main-menu.svg

@@ -0,0 +1 @@
+<svg xmlns="http://www.w3.org/2000/svg" width="48" height="48" viewBox="0 0 48 48"><path d="M6 36h36v-4H6v4zm0-10h36v-4H6v4zm0-14v4h36v-4H6z"/></svg>