Pointer subtool 'Scaling edit tool behavior'

[ollimeier]

Fixes #1679

This is a draft PR, because I have some open questions:

1. What would a good icon look like for the pointer tool with 'Scaling edit tool behavior'? (currently I used pointeradd.svg as a placeholder)
2. Any ideas to improve the dropdown UI is much appreciated. Please see the following screenshot (with the rounded corners at the beginning):

[justvanrossum]

Ha, a literal edge case :)

How about we add css that if the first tool is expanded, the lower left corner of the main tool strip is not rounded?

[justvanrossum]

Apart from the final icon design, this could be ready for review, right?