Add 'on_demand' option to interact() so that long-running functions can be started only when explicitly requested
Disable run button until the function finishes
Add tests for the on_demand option
Merge master
Change from on_demand kwarg to interact_choose function
Remove add/remove class methods,
Remove helper set/get css methods
Fix tests
Initial crack at using specific traits for styling.
Partial implementation of styles
Finished style attributes.
Fab CSS
Updated widget examples for recent changes and,
renames from earlier PRs
Fix some bugs found by the widget examples,
add margin and padding base traits,
add overflow traits to box base.
Added Bootstrap specific classes,
and fixed a few bugs here and there...
Added bootstrap3 progress bar classes
Remove references to add/remove_class methods.
Fixed rebase bugs and other bugs.
Fab CSS
Create run_kernel context manager
Simplify ExecutePreprocessor using new context manager

Closes gh-6447