Persist the last used directory between runs

[MondayHopscotch]

It should be pretty easy to drop a simple file holding the last path used by the editor so that we can load it back up each time the editor is run.

• Makes it much more convenient to load/save maps across different runs of the LevelEditor
• Will probably want to just drop the file in the working directory
• This means we also will want to add it to the .gitignore file

[Kenoshen]

#221 doesn't exactly fix this issue, but it does kind of address it