Implement ProxyCommand API

[thomthom]

Original report by me.

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So that proxy commands that mimics native functions like, Move, Rotate and Scale etc can be mapped to the same function.

#!ruby

cmd = UI::Command.new( 'Move' ) { @editor.select_tool( MoveTool.new ) }
proxy = TT::ProxyCommand.register( :movetool, cmd ) {
  @editor.active?
}