[kcm/autostart] Migrate autostart scripts to .desktop files
Currently the scripts inside of $XDG_CONFIG_HOME/autostart-scripts are not run in systemd mode (https://bugs.kde.org/show_bug.cgi?id=433987).
plasma-workspace!736 (merged) migrates them to simple .desktop files inside of XDG_CONFIG_HOME/autostart so that they can be handled in both classic and systemd boot mode.
This way they can also be seen by other desktop environments (autostart-scripts was a kde-only feature). And you can even define login scripts at the system level thanks to the xdg config hierarchy
This PR adapts the autostart KCM to use this new format. Login scripts are still visible as such (using a kde service type to distinguish them).
Instead of creating a symlink to $XDG_CONFIG_HOME/autostart-scripts, adding a new script now creates a .desktop file in $XDG_CONFIG_HOME/autostart
It needs to have plasma-workspace!736 (merged) applied and a session restart beforehand to migrate any existing login scripts.
This also separates pre-startup (env) scripts from login scripts in the UI. They were previously supported but merged in the same category.