Gradient Toolbar Improvements

UI improvements in the gradients toolbar.

The changes are:

1- Allowing to change the repeat setting when multiple gradients are selected.

2- Showing 'Multiple gradients' in the stop list when multiple gradients are selected (instead of a random stop).

3- Allowing the editing of the offset of the start/end stops.

Fixes issue #2588

4- Keeping the stop selected after the offset is changed in the toolbar (instead of selecting the 1st stop of the gradient).