#3411 - Resize sidebars using mouse

[reckart]

What's in the PR

• [x] Draggable dividers added for the left and right sidebar
• [x] Left divider disappears when sidebar is collapsed
• [ ] Dividers remember their position
• [ ] Sidebar size preferences removed from the preferences dialog

How to test manually

• Open the annotation page and try them

Automatic testing

• [ ] PR includes unit tests

Documentation

• [ ] PR updates documentation