[Enhancement] Split the 'Switch active tab' popup horizontally instead of vertically

[goran-w]

The 'Switch active tab' popup is split vertically, which is IMHO not an optimal use of 'screen estate' - splitting it horizontally would be better.

Most often I want to switch tabs (rather than open a new repo), so I want to see as many tabs as possible without scrolling.

Splitting the popup horizontally (while also making it double-width) would allow for more efficient use of the available screen space.

Switching between 'Tabs' and 'Open Repositories' with the Tab key would still work just the same as now, but we could add "2D" sideways navigation with the left/right arrow keys.