Redo activity sizing for !264

My first commit there changed sizing to set their max-height property before doing actual allocation and not after.

This mostly avoids sizing problems, except on the very first allocation when there's get_allocated_height() called on a box while it's still not allocated. Hence it returns 0.

Fixing this is pretty simple (don't rely on size of internal widgets, and only use the allocation received as a parameter), but then @tobias.bernard and @sam.hewitt told me to use aspect ratio of the screen working area. It conveniently matches the favorites allocation, so here we go.

As a bonus, the visible bottom corners indicate that aspect ratio was incorrect before:

However, the placeholder graphic needs to be redone to look good on smaller ratios. I guess drawing it programmatically would be the best, e.g. corners will have to be programmatic in the final version too.

Currently includes !264 (merged), I can rebase it to master if wanted (and merge my 2 commits in that case).