Draft: #9745: brush picker icon scale
depends on !1221 (closed) and !1198 (closed)
The brush picker visible in Paintbrush Tool Options had a few small scale issues. The external icon for the brush was hardcoded, and the internal grid icon sizes and pop-up size are small and linked. The 'Edit this brush' icon is also tiny and unresponsive to custom icon size changes.
This MR makes those elements respond to the custom icon sizes and also boosts the default size of the 'edit this' icon. A tricky one!