Issue created from #1387 and other comments:

• New brushes manager (More brush shapes and patterns #175) with preset brushes and patterns
• Possibility to Import/Export brushes
• Named brushes
• Import GG brushes (1bpp bmps)
• Brushes should be in extensions (Extensions (aka plugins, addons, packages) #1403)
• Problems displaying a lot of brushes (e.g. More brush shapes and patterns #175 (comment), ticket 3709)
• Request: switch set of shapes within one user defined brush #836

Basically the brush popup must be re-designed to simplify the management of brushes (better organization, simplify saving/loading brushes, naming brushes, switching between brushes and assigning keyboard shortcuts to brushes).