Use "Tab" key to switch focus between project tree and timeframe list

[tanius]

Keyboard-only control of tom-ui works really well already. The only missing part is a quick way to switch the keyboard focus between the project tree and timeframes list, and visual feedback about this.

Proposal: make Tab trigger the keyboard focus switch to the other part of the window, and show the selected entry in the part of the window without the keyboard focus in grey rather than blue.

[jansorg]

Done for the next update. Keeping the "light blue" and "darker blue" colors of the default Qt style for "unfocus" and "focused" states.