Group field edits together for undo/redo

[NeilFraser]

Fixes #4189.
Also use 0 instead of 0px, in accordance with the rest of Blockly and Google’s CSS style guide.

[BeksOmega]

LGTM, and procedure renaming works correctly again!

One non-code question: do you think that the custom field docs should be updated to include info about how to handle event grouping properly? It looks like outside developers need to handle this themselves because the group needs to be killed when their arbitrary widget closes.

[NeilFraser]

If developers of custom fields (that aren't descendants of textinput) don't do anything, then they will simply get a lot of ungrouped events, so it is safe. They can reduce the spam by adding Blockly.Events.setGroup(true); in their equivalent of widgetCreate_ and a matching Blockly.Events.setGroup(false); in their equivalent of widgetDispose_. But it is optional (core Blockly was missing this for about a year).

[rachel-fenichel]

Since it's release week, we should have held off on merging. I think we should revert it and hold for a week.

[NeilFraser]

Reverted.