Bring cmake installation in line with autotools

The changes in the attached archive should make the installation using CMake (more or less) identical to the one done with autotools. At least in my tests it looked fine. This is basically just for share/.

I added a new variable SHARE_INSTALL that defaults to ${CMAKE_INSTALL_PREFIX}/share but can be overwritten from the outside. This would probably be useful for Windows installs (and maybe some corner cases that Linux packagers might encounter?).

It is still lacking Inkview, man pages (someone else did that already, see mailing list) and maybe some things that I don't have the dependencies for so they are missing in my build, like dbus.