Context menus for Packages explorer

The context menu items for the Packages explorer are the standard file/folder menus, with some items that don't make much sense:

* `Packages` - edit (cut, copy, delete, rename), exclude, refactor and code cleanup. Also Tools -> Entity Framework
* `Read only` and `Modules` - has most of the same menu items, but disabled. They'll never get the chance to be enabled.
* Referenced packages - these are read only folders, so should remove all items for modification. E.g. Add, Edit (cut, copy, delete, rename), local history. Any disabled items - code cleanup, etc.. Tools -> Entity Framework. Same goes for content - they are files/folders, but read only.
* Built in packages (modules) - as above.

Note that `Assets` has the same problems as `Packages`.

This might need some changes in the Rider platform to allow modifying/replacing the context menus in the tree views.