[4D] time slider does not keep user manual change + speed value not kept

[mathildemerle]

How to see the slider problem:

• open a 4D data
• play time
• decrease Speed
• click on the slider widget and move it at an other time -> this is not kept

The Speed value is reset also for each new view and each medInria start. This is painful for users, so it could be interesting to keep the value between a medInria session but also in medInria global settings.