Allow multiple Command attributes on a single implementation

[kzu]

This allows multiple commands to use different built-in UIContext constraints, without requiring multiple classes that just inherit from each other for the actual implementation.

i.e.

class Deploy { }

[Command(.... )]
class DeployFromContextMenu : Deploy

[Command(....)]
class DeployFromToolbar : Deploy

This scenario is relevant since toolbar commands have by definition a visibility == true and would therefore turn on visibility of the command on every project. (say it's a Project context menu command).

/cc @victorgarciaaprea