[issue37902] Add scrolling for IDLE browsers

[George Zhang]

George Zhang <geetransit at gmail.com> added the comment:

I renamed mousescroll to handlescroll as it's an independent callback function and I think it fits its use case better.  I can keep it as mousescroll if you like though.

The handlescroll function is now a standalone module function in tree.py and the EditorWindow imports it for use  (instead of creating its own function).  Its signature is `handlescroll(event, widget=None)` where event is the event (yeah...) and widget is an optional argument that overrides the widget to call `yview(SCROLL, lines, "units")` on.

The second argument was added so that the nasty labels don't have to use the same function but with one line changed  (we redirect handlescroll to use `self.canvas` because `event.widget` would refer to the Label which has no yview function).

I've added tests on handlescroll with different events.  I've also added a test on multicall that checks to test if MultiCallCreator overrides yview.

Sorry about the PR closing and reopening. I was panicking about commiting more than once and saw that I should make a new branch for the patch  (instead of using master branch).

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