Update Russian translation
*Improve menus/powers.txt parsing
*Add vector of power upgrades
*Draw upgrade button if upgrades are present
Use const references
*move upgrade buttons out from power_cells.
*stub nextLevel() and upgradePower() functions.
*Implement power upgrades
*ActionBar is not updated yet on power upgrade
Load power upgrades from savegame
Greek translation update for engine.el.po
Fix nextLevel search
Fix typo
Draw next level button for powers only if current power is unlocked
Allow changing icon on power upgrade
Minor corrections to engine.el.po
Merge pull request #876 from werdanith/master

Greek translation update for engine.el.po
Remove redundancy checks. They are done on upgrade button draw.
Disable upgrade button when not drawn
Update ActionBar on power upgrade
Inspired by https://github.com/clintbellanger/flare-engine/pull/809
Merge pull request #874 from igorko/powers

Refactor menus/powers parsing. More work on power upgrades
Merge pull request #878 from igorko/master

Update Ukrainian translation
More corrections on the Greek translation
Update engine.pl.po