Use symlinks for the KF5-support sound paths

Provide the actual sound files within the sound theme structure,
and the legacy filenames (for KF5 apps) as symlinks in the
`usr/share/sounds` base directory.

This makes the theme more portable and less fragile, improving
the previous logic where the KF6 sound names relyed on the legacy
KF5 oxygen names.

Also make the installation of the legacy symlinks optional under
the CMake flag `-DKF5_SUPPORT` (enabled by default).