Disable "zoom to selection" and "home" when not available

[Anastasia Baryshnikova]

If nothing is selected, the "zoom to selection" button should be disabled (grayed out) to emphasize its role. Same thing for the "home" button (when zoomed out all the way, the button should be disabled).

[Christopher Keil]

Zoom buttons and the Home button are disabled once all available tiles are shown. Zoom-in buttons are disabled if the zoom level is maximal for the specific axis (= 1 visible tile).

[Robert Leach]

The Zoom To Selection button does not get disabled when all selected cells fill the view. The Zoom to Selection button should be disabled iff number of visible tiles == number of selected tiles and if first visible == first selected (in both dimensions).

[Robert Leach]

I have implemented a fix for this in my currently staged code. It will be in my next commit to master.

[Robert Leach]

Actually, there was still an issue. While I'd closed the ticket the other day, I discovered that the zoom to selection button wasn't getting disabled when no selection existed. That use case now works and is committed to master.

[Robert Leach]

Removing milestone: Alpha02 Release (automated comment)

[Robert Leach]

Merged to master.