Refactor context menu to make no assumptions about resource location

[tom-englert]

Header and Category in the IContextMenuEntryMetadata are resource keys, that are evaluated at runtime, where the resource location is assumed to be in the main assembly. This makes it impossible for a plugin to provide a localized header or category.

Header and category should be removed from the IContextMenuEntryMetadata and its corresponding ExportAttribute implementations, and moved to the IContextMenuEntry.
Implementations of IContextMenuEntry with the ExportContextMenuEntryAttribute should implement the Header and Category properties by return the string from the ResourceMangager of the ILSpy assembly with the same key that was provided in the corresponding ExportContextMenuEntryAttribute before.