Enhance tiddler title for system tiddlers

Now the $:/ part of the name is rendered in grey, making the main part
of the title more prominent.

@tobibeer suggested this a long time ago; good idea!
More whitespace in core templates
Rejig plugins tab of control panel

Now we divide the plugins into categories and organise the displayed
information a bit better
Get ready for a new view template segment carrying information about plugins