Revamp the waveform mode selection logic

Works in tandem with the matching [PR in koreader-base](/koreader/koreader-base/pull/259) ;).

Adds theoretical support for the KT2 & KV. (waiting on confirmation on the KT2 front).
Enable Regal waveform modes where appropriate on Kobo
Smarter logic in general (should help with UI/partial refreshes on REAGL-aware devices).

re #741