Custom data properties for elements

[exerro]

Being able to give individual elements custom data properties would be useful. For example:

local button = container + sheets.Button( 0, 0, 20, 5, "Hello!" )

button:add_data( "t", sheets.Type.primitive.number, 0 )
button:add_data( "r", sheets.Type.primitive.number, 0 )

button:set {
    x = "r + r * sin( t )";
    y = "r + r * cos( t )";
}

function button:on_update( dt )
	self:set_t( self.t + dt )
end