Enable Visual Difference mode

[Bonkles]

In v1, RapiD had a hotkey (g) that would style any changed/modified entities on the map.

The styles had three main colors for buildings, ways, and points, and also differed as to whether the change was physical (new geometry) vs. logical (new or modified tags/relations).

Idea from internal testers: also enable this mode whenever the user hovers the 'save changes' button, to give them an idea of the changes they are about to commit.