Allow plasmawindowed to use same Wayland interfaces as plasmashell

The %u in the Exec line causes kwin to not give it permissions to org_kde_plasma_window_management and others, so I had to remove it

It also seems to need a full path, so I changed it to match what happens to plasmashell's exec line with cmake

I also took the chance to change the the .desktop file name to match the rest of the reverse DNS names

I figured this is useful to those that want to test org.kde.plasma.taskmanager isolated for whatever reason