Add integration for QtQuick Controls 2 applications

[jbruechert]

~This is untested~, but should work

Fixes #129

[grulja]

I think the dependency on qqc2-desktop-style should be runtime and optional, because we don't want to automatically bring KDE Frameworks into GNOME. There should be some runtime check if qqc2-desktop-style is available and use it in such case. I would then expect to use something like Recommends: qqc2-desktop-style in Fedora packaging where we can suggest this as an optional dependency.

[jbruechert]

I now changed it to only setting the style when it is available, and added a console warning, since applications are not very usable when it is not installed. My motivation for doing this was https://mastodon.social/@tbernard/109897404484985777 btw, this is also a good example for how things look without the desktop style.

[grulja]

Btw. it also doesn't build with Qt6 and does need formatting.

[jbruechert]

Should be all fixed now :)