Implement Progress Indicators

[chrisknoll]

There is currently no approach for progress indicators in CIRCE, and ideally we would wan to use the same approach across all OHDSI tools. However, for this issue, the development task is to implement a localized (re: in CIRCE) mechanism for displaying a indicator when work is happening, and a way to signal the indicator when the work is done (so that it clears). Once we have a general approach we can create new issues to address specific work-intensive operations that require indicators. (such as saving/loading an expression, importing concept sets, etc.