Specify delay of last image of the gif animation (loop forever)

saving a gif file in ROOT has the current behaviour.

"gif+NN" sets a delay of NN*10ms to the next image.
"gif++" has to be called for the last image for setting an infinite loop, but the delay of last image cannot be specified.
"gif++NN" has to be called for the last image for setting a finite (NN repeats) instead of infinite loop. The delay between last and first image is set to funnily/buggily to NN*10ms.

So there are two things one can not control here:
(a) Setting an infinite loop with a controllable last-image delay
(b) Setting a finite loop with a controllable last-image-delay (i.e. not equal the number of repeats)

I have created an implementation to account for these new two features. I am happy to discuss or improve it and suggest including it in future versions. This is a follow-up of this post:
https://root.cern.ch/phpBB3/viewtopic.php?f=3&t=19476&sid=fbad5808b6acca008a3444d2665de9ae&p=83650#p83650

My implementation is (see attachment):
(a) "gif+NN+" for infinite loop with NN delay of last image
(b) "gif+DD++NN" for finite loop with NN repetas and DD delay of last image.

Function documentation should be also updated at several points.