[kcms/autostart] move autostart KCM from plasma-desktop to plasma-workspace

The autostart KCM affects only startplasma and has no desktop-specific dependencies or behavior (it only moves .desktop files around).

Since future changes in startplasma / plasma_session will also involve the KCM that is controlling startup files, it makes more sense to have both in the same repository.

This merge request is preserving the commit history from plasma-desktop