Allow overlay on volume of choice

[chinmaychinara91]

Based on our discussion in Slack, this PR allows user to choose on which volume they want to overlay rather than just overlaying on the default MRI.

If there is just one other volume other than the default MRI then it shows the option to overlay on default MRI.

@tmedani

[rcassani]

@tmedani, @chinmaychinara91

Check these changes, and let me know if you agree so we can merge:

The Overlay on default MRI options (MRI viewer and 3D) are now always displayed if there is a default MRI. Otherwise, the default MRI gets buried in the general Overlay menu,

Also, the Overlay on... menu is now divided in (MRI viewer and 3D). This is because one menu gets busy very quick.

Now the menu also shows the icons of the volume to overlay on

Also, these bugs are not fixed:

• Bugfix: Volumes _tissues are also Atlases, should be ignored
• Bugfix: Overlay menu did not show if there was not default MRI. This was wrong. e.g. Subject with 2 CTs, user may want to plot them overlayed.

[chinmaychinara91]

@rcassani these changes look good.