plugins/overview: Always show Desktop Bar, regardless of Virtual Desktop layout.

This merge request follows !5330 (closed)

All details in issue #240 (closed) and BUG: 475077

The code change removes the conditional filter hiding the Desktop Bar element from users with non-linear desktop layouts. It is a massive productivity helper and should speak for itself.

Let me know if there are any issues with this implementation, I have been testing for several months with a 2x2 grid to no adverse affect.

3x3 also works great, however a 4x4 grid (16 desktops) will overflow the bar on my 1440p display at default DPI ( BUG: 492373). This occurs whether or not the grid is linear, thus unrelated to this issue (could be enhanced in a few different ways, and can currently be scrolled with click+drag).

I've reached out to @vladz on Matrix to make sure it's cool that I rebased his work and we're good. Tagging @niccolove as part of the original work in Overview/Grid, and can we tag the Usability and VDG teams for review as well? (I don't seem to have ability).

Thanks for everyone's time and effort, I'd love to see this feature back in the codebase soon.