Implement XDG Desktop Portal support

[FifthTundraG]

Adds a framework for adding XDP portals and implements the Background portal for proper autostart.

Fixes flathub/com.nextcloud.desktopclient.nextcloud#18
Related: flathub/com.nextcloud.desktopclient.nextcloud#225

CC @mgallien

---

This is a draft. There are a few considerations that still have to be made:

• I have seen that sometimes the filename for the autostart file will be org.chromium.Chromium.desktop instead of com.nextcloud.desktopclient.nextcloud.desktop. It wasn't occurring on c21ff3d, which is before what seems to be a settings page redesign (which I did not notice until merging newer changes lol), so it may be related to that? This also isn't very consistent, it happened once and now it's not anymore.
• We need to figure out how we want portals to be configured
  • I think that portals should be the default, and if they fail then we fallback to legacy behavior.
  • We could also use a buildflag (something like USE_PORTALS) and make it configurable that way.
• Utility::hasLaunchOnStartup does not work (and has not worked) in a sandbox. Its usage in generalsettings.cpp was temporarily disabled, as it was preventing XdgPortal::background from being run when the "run on startup" toggle was unchecked, which meant the autostart file was never removed.
  • This check may never be able to work in a sandbox, as the portal API doesn't expose any obvious way to check if the app is already set to start on login.
• This PR has not yet been tested in an AppImage (i don't know how to build them and have not bothered to learn yet 🙃)

---

Please keep in mind that I'm quite new to C++ and insanely long buildscripts (lol), so please have grace on any silly mistakes :)