Restore the Air desktop theme

Air used to be a default theme in KDE 4 and was shipped alongside Oxygen.

The two are complementary - Air provides a light theme experience, Oxygen a dark theme one.

Oxygen was moved to this repo, but Air had been removed from libplasma beforehand.

Both themes still work so there's no reason not to install both of them and give users an option to have a light panel when using Oxygen design.

This commit restores the Air theme in the state it was before its deletion.