Rework main UI to add a sidebar

It probably doesn’t make much sense to review the diff here, as a lot of them are XML changes which will make little sense. I have tried to split the changes up into commits to add a bit more structure to the diff, but I didn’t find it particularly easy. The code will almost certainly not compile on any of the intermediate commits.

Here are some screenshots of the end result. Please review on the basis of how well these implement the designs, but consider whether review comments should best be fixed before this MR lands, or as follow-ups. Since this MR touches a lot of core XML UI files, it will bitrot quickly.

The changes include some adaptive support for a small main window, but this is not intended to be complete or polished. That can happen in follow-up issues (which I would encourage people who care about adaptive UIs to work on; it’s not a priority for me).

One difference to discuss from the design is the icon for the ‘Updates’ entry in the sidebar — I couldn’t find the icon used in the design, so used a different one instead. It might need to be changed.

Finally, the designs use a limited set of categories (Create/Work/Play/etc.). I haven’t included changes in this MR which change the set of categories, so the existing set continue to be used. The idea with that is to avoid holding the sidebar changes up due to bikeshedding the list of categories. I’ve filed #1207 (closed) about the categories.

Helps: #1111 (closed)