drawable: Recompress the PNGs.

For this task, I used the command `advdef -z4 -i1000 *.png`.

Signed-off-by: Rogério Brito <rbrito@ime.usp.br>
Reformat XML files for legibility.

This makes the code more readable (or, put another way, more prone to
external contributions), at least as far as I am concerned.

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