Re-add autostart
This is a partial revert of b1c575ce that brings back autostart for the primary usage mode (i.e. as an onboarding wizard) but in a technically simpler form:
- We now use single config file key that encompass both skipping and completing the wizard, which are the same from the perspective of whether we want to autostart it next boot or not.
- We now decide whether or not to autostart on the next boot by using an X-KDE-autostart-condition key in the .desktop file, rather than conditionally deleting the autostart file
We still need to figure out a way to open the app in upgrade mode following an upgrade to a new major Plasma version.
This does not seem to work and I could use help figuring out why. Even manually putting the .desktop file in /etc/xdg/autostart without the X-KDE-autostart-condition line doesn't make the app open at login for me.