Added some convenience functions to update UIElements

Added some convenience functions to update UIElements without having to have references to them, i.e. by group name. Particularly helpful for button groups.

Example: say you have multiple buttons in a "colors" group that are mutually exclusive. You set which one is pressed by default when setting up your UI, but later on you want to set it to something else programmatically, without having to maintain a bunch of UIElementRef's.

```c++
// initially button with name "1" is pressed
mParams->addButton( "1", std::bind( &UserInterface::setColorMode, this, 0, std::placeholders::_1 ), "{ \"stateless\":false, \"group\":\"colors\", \"exclusive\":true, \"clear\":false, \"pressed\":true }" );
mParams->addButton( "2", std::bind( &UserInterface::setColorMode, this, 1, std::placeholders::_1 ), "{ \"stateless\":false, \"group\":\"colors\", \"exclusive\":true, \"clear\":false }" );

// later on you could do this
mParams->selectGroupElementByName( "colors", "2" );
```