gradient end stop, "insert new stop" button might need to be disabled

[Cojnel]

When the end gradient stop is selected,
then the "Insert new stop" button should probably be disabled,
because it's only possible to add gradient stops with that button,
when the first or a mid gradient stop is selected.

Tested with:
r12841 on linux mint 15 cinnamon 32bit

[su_v (suv-lp)]

Reproduced with r13502 on OS X 10.7.5

[Wong Cho Ching (chwong249664)]

I have removed the easy-fix tag. Due to legacy C code, this bug is pretty difficult to fix.

In`widget/gradient-toolbar.cpp`, I tried adding `gtk_action_set_sensitive(GTK_ACTION(add), FALSE);` in `gr_tb_selection_changed()`. Holy crap! After compling and running, the add button isn't even disabled. Appearantly, the sensitivity of the button is set to TRUE in somewhere else.