Several changes to hider building/upgrade system

* Stats are now held in upgrade ability, rather than a separate stats
ability
* First ability slot leads you to the building create menu, which
contains all the buildings you can create (swapping abilities), and a
return to main ability
* Second and third ability slots are now empty and reserved for skills
learnable further in the game (TODO)
* Fixed a bug when only one created tower could be salvaged
* Fixed a bug when salvaging a mine decreased your income by double the
mine's gold per tick