Fix memory errors in Subsurface

[Thiago Macieira]

This patch series fixes the memory errors found by Lubomir's run with
Dr. Memory and my own running with Valgrind after fixing all issues he
found. The majority of the leaks are not real, but are fixed anyway in
patch #4 so we can actually read the results.

After this patch series, Valgrind reports only the following errors in
a Subsurface run:
 - WebKit memory errors, probably due to the JIT
   (even with --smc-check=all-non-file)
 - a TON of memory leaks in Marble and its plugins. There are so many
   leaks there that it's not even fun.