Reorganise (again) the build system

Reorganise (again) the build system

- Require at least Qt 4.6 and KDE 4.4.
- List dependencies when configuring.
- Cleanup Qt dependencies handling. There is no need anymore to
  include specific Qt libraries in target_link_libraries(), as
  QT_LIBRARIES will be adjusted according to the modules requested in
  find_package().
- Move from platform checks to feature checks (should solve problems
  on GNU/kFreeBSD). Some Mac OS X checks are still there: I'll see
  what to do with them.
- Make Video4Linux support optional.
- Last (and least), tidy up CMakeLists.txt.