Split "Theme & Icons" into "Themes" and "Icons"; "UI Libraries" into "Control packs" and "Controls"

[Athari]

The most useful sections are currently too hard to navigate as they clump everything together. I suggest splitting combined sections:

• Themes: themes for controls; can include icons and everything else.
• Icons: actual icon packs; can include controls and markup extensions for displaying icons and other icon-specific utilities.
• Control packs: big control packs with 10+ controls.
• Controls: individual controls or highly specialized small packs of like 3 controls.

Reasons for the change:

• When I'm looking for a theme to restyle standard controls, I'm not interested in icon fonts.
• When I'm looking for icons, I'm not interested in restyling all my controls.
• When I'm looking for control packs, I'm not interested in individual controls, as it's usually a good idea to limit the number of big packs to exactly one.

There may be borderline cases I'm not aware of, but I think vast majority of packages cleanly fit into these categories.

If you approve the change, I'll submit a pull request.