Add function to get a list of matching commands

Add a function get_matching_commands in engines that allows to parse a setting of the following format:

menu_favourites:
- {app_instance: tk-multi-workfiles, name: Shotgun File Manager...}
- {app_instance: tk-multi-snapshot,  name: Snapshot...}
- {app_instance: tk-multi-workfiles, name: Shotgun Save As...}
- {app_instance: tk-multi-publish,   name: Publish...}

and extract all the commands that match the specified app instances and command names ('' being all commands from that app instance).