Closes #258

In addition, it removes unnecessary function type casts like:
  (GDestroyNotify) g_object_unref
  (GDestroyNotify) g_free

To detect the memory leak using valgrind:

export CFLAGS="-Og -g -fno-omit-frame-pointer -fPIE -pie"
export CXXFLAGS="-Og -g -fno-omit-frame-pointer -fPIE -pie"
./autogen.sh --prefix=/usr
make clean
make
sudo make install
valgrind --tool=memcheck --undef-value-errors=no --leak-check=full --log-file="report.txt" /usr/bin/mate-disk-usage-analyzer

report.txt content:

==16061== 6 bytes in 1 blocks are definitely lost in loss record 174 of 14,390
==16061==    at 0x483880B: malloc (vg_replace_malloc.c:309)
==16061==    by 0x56B10D7: g_malloc (gmem.c:99)
==16061==    by 0x56CAAEE: g_strdup (gstrfuncs.c:363)
==16061==    by 0x56E3743: g_variant_dup_string (gvariant.c:1533)
==16061==    by 0x5509DE6: g_settings_get_string (gsettings.c:1795)
==16061==    by 0x115664: initialize_charts (baobab.c:1185)
==16061==    by 0x11640C: main (baobab.c:1307)

  PNG icons are created if they are not present in icon folder.

  To remove png icons:
    make -C baobab/pixmaps clean-png-icons
    make -C logview/data/icons clean-png-icons

  To build png icons:
    make -C baobab/pixmaps build-png-icons
    make -C logview/data/icons build-png-icons

This removes baobab/pixmaps/shot.png file, use applets-screenshooter
named icon (used in mate-screenshot).

full stop at end of sentence in license paragraphs