libdeng2: Improved Range template

More operators for Range and a substring method that uses Range as
the argument.
libdeng2|Log: Improved styled log entry output

The entry level can be omitted as that information can be visualized
using rich formatting. Also added level/message styles for the lowest
levels (debug and trace).
libshell: Cleanup
libgui|Font: Rich formatting of text

The format escape sequences in styled text can be interpreted using
RichFormat. RichFormat can then be given to Font, FontLineWrapping, and
GLTextComposer to modify the appearance of the text.
Client|Widgets: Use rich text formatting in the LogWidget

Style markup in the log entries is now taken into use in the
LogWidget. Font::RichFormat is used in FontLineWrapping and
GLTextComposer. Lines can also be indented according to the indentation
markers in the rich text.

Todo: de::Font needs to know which colors to use for the rich palette