Fix plot dialog crash when changing plot type.

The problem was the fix with in commit
4c6aa2f0373774639892a5b9304c93eaed857a2a. It caused an infinite recursion
on the plot dialog where the change signals are directly connected to
the plot updating. The signals are now not dispatched when the text
is interpreted for the value call. Refs #7084
Merge remote-tracking branch 'origin/bugfix/7084_crash_plot_dialog'