Settings button now has proper background layer (baked in)

[Spengreb]

Fixes transparency issue around the newly introduced settings button in my previous PR #329

I put the gradient background into the settings image manually by taking a screenshot of the gradient and applying the image on top in GIMP.

Notes for anyone else wanting to make UI changes:

The background gradient only extends through the middle of the header, the LUPPP logo and OPENAV branding image contain the gradient of the background baked into the image itself. There are no layers going on here.

When I originally introduced the settings button it did apply transparency correctly but also pushed the background gradient to the right, so instead of the transparency being the background gradient it's just the black default background.

Maybe one day we can add layers here so this becomes easier... and maybe one day we should switch to .svg images rather than these funny .c images!