Strategy for testing keyboard focus on Linux

[corranwebster]

This comes from issues testing #862, but also has relevance in the Tasks infrastructure where focus is used.

On linux, keyboard focus is different from Mac OS and Windows, and it is harder to guarantee that a widget has focus as expected throughout a test. Focus is only True when the window containing the widget is active, and in some cases systems may be configured so that keyboard focus follows the mouse pointer. In effect, keyboard focus is system global state that we can't easily control, particularly if there is an active user working on the computer.

It would be good to come up with a strategy that we can use for testing that focus-based methods are working as expected.