Create mixin that tries to execute primary node command

ve.ce.ClickableNode:
 * Attempts to execute the node's primary command on dblclick.
 * Nodes this is mixed into must emit dblclick on their
   overlayed elements

ve.ce.Surface:
 If node is focused and the enter key is pressed, the node's
 primary command will be executed.

ve.ui.CommandRegistery
 * Create method to get the node command

Bug: 50996
Bug: 54827
To be used by: I6b45018bdb7
Change-Id: I8df07f413d9e295b960b7d0c82224aa37fdd637e
Merge "Create mixin that tries to execute primary node command"