GuiTabular: disabling is now done on textChanged()

GuiTabular: disabling is now done on textChanged()

Before, it was done on editingFinished(). The new behavior is more
intuitive because it is easier for the user to see how editing the box
is connected with enabling/disabling the other widgets. Before, the user
would not get instant feedback and would have to click away before the
connection is revealed. This new behavior is less efficient, however,
because checkEnabled() is called after every keystroke.