Added an 'Advanced' settings checkbox that allows the user to set a given
VM as template fo dispvms. It sets both the template_for_dispvms property
and the appmenus-dispvm feature - instead of delving into particulars
of how and which should be set, the user can just turn off the
whole setting if they want to make some changes in the vm and then turn
it on again.
fixesQubesOS/qubes-issues#4004