wip: oop-ify app

[GeopJr]

The way Collision is currently built, it only has 1 window instance and all logic handles that instance's widgets.

This PR changes that, allowing us to create multiple windows / open multiple files at the same time.

Collision

• Move from ui files to creating the widgets in code
• "New Window" menu item
• Remove welcomer file chooser (use the main one instead)
• Organize views, widgets, functions

nautilus extension

• Handle multiple files

This is not done, there's still invalid memory issue when opening way too many files at the same time (wip) and specs need to be re-written.

fix: #104