Reassigned tower hotkeys

* 1 - Melee
* 2 - Ranged
3 - Special
4 - Heal
5 - Aoe
6 - Boost
* new or changed hotkeys
Adds GameEra for the era of the current game

The GameEra and GameLevel is initially set by the AI of each playable
level at a priority of -100. Scripts that run after this can rely on
GameState.GameEra and GameState.GameLevel to reflect the era and level
of the level being played.
UnitPrefabs are lazily loaded on Era change

Fixes - Prehistoric sprites are persisting through different eras.
Added cooldown bars to AbilityTowers