Introduce new Progress GUI class

This dialog will serve as a replacement for existing calls to the Anki
main window's progress object, which was not exactly intended for this.
Reintegrate BrowserGenerator w/ Progress

Removes old calls to the Anki main window progress object and instead
instantiates and manipulates the new Progress class.
Add message about rolling back notes

If the user cancels their processing, provide a message that explains
how to undo what has been done.
Change undo menu

Because the user can now cancel processing, we do not necessarily know
the effective note count ahead of time.
Documentation update for Browser Generator

Mentions the options to cancel and undo processing.
Add detail field to generator progress dialog

This new field will be usable for displaying the phrases as notes as
processed.
Display each phrase during mass generation

This new functionality will be helpful for users who might accidentally
choose the wrong fields, allowing them to cancel the generator and start
over (perhaps with an undo).
Merge branch 'add/cancel-gen-34' into develop

Adds support for cancelling the mass generator and being able to see the
notes as they are individually generated.

Closes #34.

Conflicts:
	addon/awesometts/gui/generator.py (adjacent changes)