Make Daïza work with Godot 3.0.2

[fleskesvor]

There's a lot going on in this PR, but the bulk of the changes are related to the new project structure that's been discussed in godotengine/escoria#44 and the #escoria channel on Freenode, and was implemented by godotengine/escoria#61. The goal was to split up reusable scenes into a hierarchy of rooms, actors, items, etc. and make it easier to keep the framework up to date while developing a game in a sub directory, which can be kept under version control separately from the framework, eg. as a git submodule. I don't think we're there yet though, but it's hopefully a step in the right direction.

The PR also removes some redundant files, fixes scriptable background music, reorganises the project settings under a new "Escoria" category and has several new features. There are, however, some known issues:

• Window resolution is off, making verb menu and inventory partially render outside of game window
• Sorting of UI elements is wrong. Dialogue appears under foreground elements
• Branching dialogue doesn't work, and crashes the game when talking to the chameleon. See: godotengine/escoria#86
• Click masks still don't work. I've tried adding and re-importing png files as black and white Bitmap resources as instructed by godotengine/godot#15524 (and the editor), but that seems to disable interaction completely

[fleskesvor]

Branching dialogue has been fixed upstream. I'm closing this PR for now, and will re-open it when it's in a better state.