Make the bevy_editor_pls dependency optional

[InnocentusLime]

Currently the plugin relies on the platform provided by bevy_editor_pls. However, it seems that making the plugin work on its own is:

• Not that hard to implement (assuming egui is still an required dependency)
• Makes the editor more usable (not all bevy developers are using bevy_editor_pls)

Tasks:

• Try to isolate the code from getting the &mut World reference
• While we are abstracting away from the from bevy_editor_pls, the whois tool can offer some tighter integration with it (clicking on the tile can force the inspector into viewing the tile entity)
• Consider placing bevy_inspector_egui behind the optional feature gate too
• Redesign the API to make it easy-to-use without bevy_editor_pls (probably just make State into a trait)
• Create the API that allows adding the plugin without bevy_editor_pls
• Integration with egui_dock
• Make some of the State API public