feature/theme delegating the renderer to a default implementation

This change delegates the render of ui_list ui_controls and ui_input to their equivalent member functions in a class instantiated as a singleton.  Themes can then register their own implementation with the singleton (in lieu of a container) in order to modify the render implementation.

A sample [bootstrap 3 theme](https://github.com/cambell-prince/frontaccounting-theme-bootstrap) has been implemented that demonstrates the technique.  This theme is intentionally close to the basic bootstrap.