feat: enable custom sections

[sircharlo]

To do:

• Create dialog to show existing custom sections and allow adding new ones
• Set random color when creating new section
• Add color picker
• Modify logic to include custom sections in the media list
• Allow sorting of custom sections
• Allow drag-dropping to and from custom sections
• Properly persist drag/drop reordering and list changing for custom sections
• Show media items in standard "additional" section if the referenced section doesn't exist
• Allow renaming the standard "additional" section and changing its color
• Enable heading/section import from jwpub file

[ux-git]

@sircharlo if you'll need any other graphics, colors, states, icons, windows, etc – just ping me.