doxygen: png -> gif for images
doxygen: png -> gif for images

This reduces the amount of output in docs/ from 17G to 10G for me. No
big surprise, inheritance diagrams use only a few colors, so gif's
indexed colors work here nicely.

Thanks Christian Lohmaier for the idea.

Change-Id: I23db0c1a4dc7410cb3eb1ae99044962959fac28a

JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core