Proton Compatibility

[soup-bowl]

Split into two segments, one with the problems that can be seen with browser.proton.enabled, and ones visible regardless. Latter is the priority for achieving MVP.

Changes are tracked in Photon branch (aware the name is wrong).

Constant.

• [x] Misplacement of Tabs bar.
• [x] New element on bookmarks bar causes distortion.

Proton-related (aka, disappear when browser.proton.enabled is on).

• [x] Tab size exceeded tab bar.
• [x] (technically without) Back button is sized inconsistently.
• [x] URL bar elements break out of their confines.
• [x] Missing icons in the Application Menu (might've been a Nightly bug).

[soup-bowl]

The even-sized UI button conditions for pre-proton Normal and Touch densities have been removed, as I was unable to get them to work correctly with Proton conditions. I tried a various different medley of conditions using uidensity and proton, but nothing seemed to work without the Proton. Since this will take over, I've removed it in favour of Proton, since Compact still works fine.