Give tiles a meaningful empty state

This will be used to prepopulate grids with empty tiles.
Actually enforce squareness of popular tiles

This was somehow forgotten, and worked just by accident.
Make the size of featured categories accurate

We want to use the size when prepopulating grids,
so don't lie about it.
Show the window right away

If yum holds the lock, we may otherwise ending up not
showing anything for minutes, which is just broken.
Avoid undue jumping in the overview page

Populate the featured and pick sections with empty tiles
initially, so we can fill them in without having things
jump around too much.
Prepopulate category grid

Show empty tiles in the category page as well, and replace them
with the real thing when available. We use the size of the
category to know how many empty tiles to put there.

At the same time, show all non-empty subcategories. An arbitrary
minimum size does not really make sense, and we always want to
show the featured category which may be smaller than 4.