Add automatic window layouting, per-instance user directories, more convinient custom arguments and editor buttons

[dreadpon]

I needed this plugin for a project I'm currently working on, but some of the things were a bit too manual, so I optimized them a bit.

These edits include:

• Automatic window layouting in a grid (custom distance between windows is still an option)
• Per-instance subdirectories for user:// path. Necessary when you want to store per-client data such as cache or client-side credentials. This addition allows to prevent overwriting these files if they share the same name (see README.md and instance_setup.gd for instructions)
• Better command line arguments, where all arguments that are passed to main scene are inferred by all instances and additional ones can be supplied via a dictionary
• Improved editor buttons to give them native look and functionality