Improve tabs

[OfficiallyM]

Requires #37 first.

Improve the tab system to use an abstract class.

Probably needs the following properties:

• string - Name
• bool - HasConfig (allow custom config panes?)
• void - Renderer (called when tab is active)

This would allow other mods to add new tabs, allowing for modder controlled mod integration with the spawner

[OfficiallyM]

Initial implementation added in 0c13d4a818ff61a153898e1a1f42616cad69e3fa

[OfficiallyM]

Further improvements:

• Show list of all external tabs in settings with their source mod name
• Allow external tabs to be disabled from renderering

[OfficiallyM]

Delaying until next version