Move towards a more logical layout for KStars' source tree.

The kstars/ directory will hold code that's tied to the GUI,
while the src/ directory will also have other things like the
engine, etc which are not.
Move the HTMesh out of kstars and up to src/
Move datahandlers/ to src/
Move the data directory to the top level.
Move the icons folder into the data directory.
Move dependency checks to root CMakeLists.txt