*Implement power upgrades
*ActionBar is not updated yet on power upgrade
Load power upgrades from savegame
Fix nextLevel search
Fix typo
Draw next level button for powers only if current power is unlocked
Allow changing icon on power upgrade
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
Use upgrade buttons only in correct tabs
Remove redundancy checks
*Allow more functions that deal with power cells to accept both power index and vector
*This allows using upgrade vector when needed (for tooltip generation and button rendering)
This was removed by accident
Merge pull request #879 from igorko/power_upgrades

Power upgrades