Hides the brush asset shelf by default in weight paint, grease pencil
weight paint and grease pencil vertex paint mode. These only have one
bundled brush per tool available.
After bringing back some tools for brushes in certain modes
(3798852071), we want to hide the asset shelf in modes where there's
only one brush bundled per tool. While having these tools support
brushes can be useful, we don't expect to bundle more brushes soon. So
avoid a mostly empty asset shelf for a single brush by hiding it by
default and keeping it hidden until the user expands it (bf52f6f723 made
it possible to remember the visibility per mode).
Part of #128066 and #116337.
7191608a98