Minor tab/focus fixes

[tom-seddon]

Fix some minor issues with the docking system that I noticed when fixing various focus bugs in my program's UI.

• Draw focused dock's tab button in a different colour
• When clicking on a dock's tab, give the dock the focus

I picked tab button colours based on the default blueish colours - it's possible these could need tweaking a bit.

[Flix01]

Thanks!