*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.
Update engine.be.po

Translated new strings
Greek translation update for engine.el.po
Merge pull request #875 from mikhailSaTuRn/patch-1

Update engine.be.po
Update Ukrainian translation
Minor corrections to engine.el.po
Merge pull request #876 from werdanith/master

Greek translation update for engine.el.po
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