UI: Support viewing state objects from any GL context
* Add a combo box above the state object tab control which contains all available contexts.
* Text in combo box shows context handle, if it is current, and a list of shared contexts.
* By default the current context will be selected in the combo box.
* Selecting a context will update the tab controls to show the state and objects within (or shared with) that context
* Modify the state tree so that it only displays the state of the selected context. Objects shared from a different context will NOT show up in its tree.