Fix inconsistent legacy button mappings

[C0rn3j]

• 7ef96e0 - scc/gui/creg/constants.py at least maps LPAD press as "LPAD" instead of the vars that are used by the Deck, presuming another legacy incompat - probably because LPAD should be DPAD here.
• 7ef96e0 - DPAD is currently contained within LPAD too somehow.
• 7ef96e0 - STICK/STICKPRESS should be changed to LSTICK/LSTICKPRESS everywhere.
• LEFT and RIGHT seem to mean "LPAD" and "RPAD" but are used ambiguously.
• LPAD.svg and RPAD.svg are specifically for Steam Controller (2015).