theme: default: Micro-optimize the SVG files.

A lot more can be accomplished if we create groups, which means that a bunch
of the attributes will be shared between elements.

Signed-off-by: Rogério Brito <rbrito@ime.usp.br>
theme: default: Use a more compact copyright statement.

Using the `scour` tool, we may get a lot of savings for the SVG files
(taking care of preserving the IDs, if we intend to manipulate the SVG files
via JavaScript).

Signed-off-by: Rogério Brito <rbrito@ime.usp.br>