v0.1.3 -- Progression changes and refactoring

I'm finally back to working on this!

I spent a significant chunk of time over the last 3 days refactoring code and fixing bugs.

The main observable changes to the game are:

• You no longer lose your items or equipment upon returning to town.
• You now lose unspent experience points upon entering the tower (to match how gold works)
• Tutorials and dialog boxes have been updated to reflect the above changes

These changes collectively result in a much shorter playtime. Since you don't have to re-purchase all your upgrades each time you return to town, your power level increases more quickly. I was wary of making this change at first, but I think it will be good for the game in the long run, because it will let me have lots of other side content that players would be willing to spend gold and experience points on. You could spend money on improving the town, getting collectables, completing side quests, etc. That will come far in the future though.

Under the hood there were huge changes to the "persistence" system, which manages all data that needs to be shared between scenes or accessed by multiple nodes in the same scene. There's a lot less reliance on signals, and the whole thing feels way cleaner.

There's no new content in this build, but I do feel a lot more confident about the codebase, and I think I'll be able to proceed without feeling too overwhelmed.