Fix gradient toolbar

Alright, summary of the problems:

Issues:

1. Empty first gradient (should always have text shown). empty first stop (when selected via canvas)
2. Repeat disabled
3. Reverse doesn't work

Solutions:

1. Fixed by removing check for -1 in combobox value. Don't know why, but -1 is valid input, no reason to block. In terms of introducing more bugs, the custom combo box is only used in the gradients toolbar and in various unit trackers (which don't clear their options).
2. Widget being enabled/disabled when different/child from the one that is initially disabled.
3. A recent migration, appended the stops instead of prepending them. Adds them correctly.
4. Also made the text of Repeat be disabled with the combobox.