A better categorisation of themes

[Kazhnuz]

There is some problems in the current categorisation of themes. First, the "metallic" category is pretty precise for what it could be (something for skeumorphic theme, compared to flat theme), and some theme could be needlessly out of scope of this category. Secondly, I wonder if for some themes like GNOME OSC ( see #52 ), a special category for themes collections, or "look-alike" themes could be great ?

So if there isn't any disagreement, I propose that the Categorisations is changed to the following one :

• Look-a-like themes
• Material Design
• Flat Design
• Skeumorphic
• Other

I'm marking the issues as help needed as I need help on the wording, though. Maybe if many "retro" themes works great for GNOME, a "retro" category could be great ?

[Kazhnuz]

See #79 too. So the new categorisation would be

• Upstream-based
• Look-a-like themes
• Material Design
• Flat Design
• Skeumorphic
• Other

[Kazhnuz]

Note : Another thing would be to fuse the Cursor/Theme/Icon category into one big "Look-and-feel" category, to make things easier.