Completed refresh-execp

See merge request o9000/tint2!46

Fix imlib image leak

Closes #704

See merge request o9000/tint2!39

Double-adding the image caused imlib's reference count to increment
twice, requiring us to free it twice to actually release the cached
image, but we only ever free it once.

Fixes #704, likely #721 (leak is not present with -DENABLE_RSVG=OFF),
possibly #650 based on connection to execp, maybe others since this was
introduced nearly 6 years ago in 1d02b858 in launcher-specific code.

Fix crashing when a window icon is large

See merge request o9000/tint2!35

Enhanced example for memusage executor

Closes #787

See merge request o9000/tint2!38

If memusage example got stdbuf -oL awk, why further examples
can't use same?

It seems, html help already had one free instance with -s option for persistance,
though still used inner non-persistant chain for humanized output.

If an icon of a window is large tint2 crashes with segmentation fault, I guess because of stack overflow. This changes the allocation of icon_data from stack to heap.