Assembly Definition refactoring and management.

Creating and managing assembly definitions inside Unity is very clunky and a lengthy and annoying process, to the point that I do it all in a text editor instead. Therefore, I think it would be productive to add some enhanced support in the plugin for management and refactoring tooling to greatly facilitate working with asmdefs.

The main one I'd be after is a refactor option for generating a new assembly definition based in the selected folder in the solution explorer. Of course, it'd also display a dialog with a picker to select the references.

Some extra ideas that would be nice as well, but aren't a deal-breaker...

- The GUI should figure out on its own and check the assemblies that are gonna be needed by default when the dialog opens, or at least check the ones from the assembly from which it's being created.

- Displaying a gui for asmdef files which facilitates editing by checking/unchecking wanted/unwanted assemblies. Actually, it would probably be a better idea to just access a dialog windows from a context menu item on the selected assembly in the solution explorer.

- Marking references that aren't being used for anything inside the assembly as such.