Igcs 79 tab color

* made tab bar transparent (and modified all screens to have matching black backgrounds)
   - found opaque black tab bar was insufficiently differentiated from the rest of the (black) views

* adjusted mission editing bar button padding (as previously discussed)

* toned down the "jarring" follow me UI with some alpha

* adopted an application wide tint color
   - chosen to match with Artificial Horizon tick colouring... looking forward to some bike shedding on this one! :-)