Add multi-document support

[meh2481]

Add support for working on multiple sheets at once (i.e. tabbed document support). Will need to update UI fields when switching between tabs & abstract sheet-specific logic into its own class if it isn't already.

Look into whether this will require adding UI-specific logic to sheet (such as animation type), or whether this can be pulled into a UI view for the sheet, which can also update the main UI.

Likely will require drastic UI restructure. May be worth properly following MVC now.