Files in src/util should be in namespace mv::util

[alxvth]

Currently, some files in src/util define classes that are in no namespace, others use mv, mv::util or mv::gui.

I think it would be good to place all utility classes in to the mv::util namespace.

At least, we should place the two classes that are not in any ManiVault namespace yet (Timer and AbstractItemModelTester) into mv::util. Especially the Timer class has a very generic name and that might result in linker errors for plugins that also define a Timer class.