CMake: enable Qt5 apps by default; provide instructions if Qt5 not found

This hopefully solves a set of common problems:
- Usually people want to build with Qt 5, but it is not enabled by default,
  and instructions how to enable it were not easily accessible.
  Hence rather just enable it by default.
- When Qt 5 is installed in a non-standard location (as usual nowadays),
  there is a tricky procedure to get it found by CMake.
  Hence, when Qt 5 not found, automatically provide instructions how to
  do that.
- When Qt 5 not found, instructions are automatically provided how to
  disable it if not needed.