Fix cursor left/right navigation in overset, underset, and stackrel

Fix cursor left/right navigation in overset, underset, and stackrel

Before this commit, navigating with the cursor was visiting either
the nucleus or the script depending on the direction (left or right)
of the cursor movement. Now the 2.3.x behavior of always going through
the nucleus is restored (at least for overset and underset, as stackrel
seems to behave oddly also in 2.3.x).