deprecate heading cells in UI

- removed from Kernel Menu
- Main toolbar warns via shouty dialog
- keyboard shortcuts still work to make headings in markdown cells, with no warning
Merge pull request #6846 from minrk/deprecate-heading-ui

deprecate heading cells in UI
distinguish capture_output from buffer_output

allow capturing output without silencing it,
which is useful for slimerjs,
which requires looking at output to determine failure.
Merge pull request #6860 from minrk/slimerjs

distinguish capture_output from buffer_output
initial take on promises
Initial stab at adding promises to the widget framework.
Fix view rendering order.
Add Promise support to models.
Use load_class in comms instead of callbacks.
Proxy console.error calls
Simplify code by using Promises in a better way; try_load -> load
More simplifications due to promises
Add some error handling for creating views and models
Better error messages with correct stack traces
Dictionary key/value typo
Load the utils module
Fix some this vs. that errors, as well as returning some promises to wait for fulfillment
Make display also pend on set_state.
Add a WrappedError class
Finished adding error handling.