Memory improvements and Memory Label

https://med.inria.fr/redmine/issues/1765

This PR adds a memory label in the bottom right corner which gives information about the current memory state of the computer. 
We have rewritten some of the functions in medDataManager. This PR might solve problems encountered with the cache.
It also improves the memory management of the software by removing two allocations that are considered as useless (no regression detected). I advise you to test it with large images to clearly see the difference.

Reviewer : Olivier, Guillaume
Merger : Flo