Meta mods

Hi, good evening :)

Relatively small update, but an important step to a nice gameplay loop: mods are here.

Mods are acquired every time you beat a map, and come in 5 tiers.  They can be upgraded to the next tier after unlocking it by beating specific maps on corresponding difficulties, aka beat easy for tier 2, or hard for tier 4.

Every tower can be equipped with 3 mods, so play around and find what works best for you.

For now there are only 7 mods, linked to the 7 Simple maps. These are also still relatively bland I'll admit, being simple stat increases. But now that the system works I can add more interesting once in the future, and for now these add at least some customization and meta progress.

In true me fashion, the pages for mod management and equipping them on towers have absolutely horrific ui, sorry and please live with it.

Please let me know what you think about the amount of grind these introduce if you have the time. I want to stay somewhere nice and between being immediately done making replayability useless, and the grind turning into a second job xD.

Also did a few minor tweaks and fixes, including giving everything a base range to make seeing what is selected clearer, thanks Matt :D