Convert UI to Dalamud WindowSystem

[chirpxiv]

Refactors PluginUI to make use of Dalamud's WindowSystem.

This provides the following benefits:

• Simplifies ImGui windowing, reducing the amount of boilerplate code.
• Encapsulates windows into individual classes, making them easier to modify and maintain.
• Prevents windows from being dragged off-screen.

This should function near identically to the previous implementation provided I didn't miss anything in the process.