Add TabFocus to focusPolicy instead of overriding it

This is a new attempt at solving the issue in !203 (merged) without causing the compilation error. This is the code I originally wrote in !203 (merged), which I later changed according to the suggestion of a reviewer without checking (by mistake) whether the suggested code actually worked.

The reason I used StrongFocus instead of just adding | TabFocus (aside from the compilation error) is that, unless I'm not understanding the documentation correctly, ClickFocus|TabFocus is not a valid value for Qt::FocusPolicy which, unlike other similar enum, doesn't have a corresponding QFlags.