* Layout toolbar using CSS grid, introducing left and right trays
The trays provide a space for add-ons to introduce their own widgets to the toolbar without interfering with each other.
* Align tray items to the top
* Move absolutely positioned add-on items to right toolbar tray
Workaround that fixes breakages in add-ons like AMBOSS, Study Timer, and potentially others that currently still inject absolutely positioned elements into the toolbar using `top_toolbar_did_init_links`.
* Account for add-ons that add manual padding (e.g. Study Timer)
* Add docstrings and slightly refactor
* Tweak item alignment
* Introduce hooks for extending left and right toolbar trays
* Assign CSS classes to all tray items
* Add disclaimer on transitional nature of new hooks
