CMake: Add more/better GNUInstallDirs variables usage

1. Use more of GNUInstallDirs' installation location variables: MANDIR, BINDIR
2. Don't assume that prepending CMAKE_INSTALL_PREFIX to any of the location variables makes sense. There is no guarantee that install locations are relative to the prefix (see CMAKE_INSTALL_<location> documentation) - forcefully prepending the prefix to them can generate nonsense. CMAKE_INSTALL_FULL_<location> is what should be used to get a guaranteed-absolute version of the locations.

Same as in libusermetrics!12 (merged): We (downstream) would prefer the usage of CMAKE_INSTALL_DOCDIR for controlling the documentation install location, rather than binding the location to CMAKE_INSTALL_DATAROOTDIR. This would however come with a change of the documentation path from doc/liblomiri-thumbnailer-qt to doc/${PROJECT_NAME}, with PROJECT_NAME being the top-level lomiri-thumbnailer. The old path should be preservable by adding a new project() call, but I'd prefer your input on any of this before making any decision.