Add TextEditor::withGroupingInterval

This method temporarily instructs the editor to apply undo grouping with
a given interval. This way, undo grouping can be made optional without
adding optional arguments to every buffer manipulation method.
Use undo grouping in editor command listeners
Remove TextEditor::withGroupingInterval

Just use ::transact
Update signature of calls to TextBuffer::transact
Don't wrap undo/redo calls in transactions
Make undo grouping interval configurable
Reduce ambiguity in config description
:arrow_up: text-buffer@3.6.0
Merge pull request #4071 from atom/mb-editor-grouped-undo

Add grouped undo for all text editor commands