PageHeadSections: changes selection when adding/removing items

[Sam Bull]

When adding/removing an item from the model, the item selected by the PageHeadSections changes. Selecting the next item along when removing an item before the selection, and the previous item when adding an item before the selection.

Note that this behaviour works correctly in the Picker component (except for bug #1344883). Basically, the element should pay attention to changes in the model and update it's selectedIndex appropriately in order to continue selecting the same element after updates.

A minimal example is attached to demonstrate the problem.

[Tim Peeters (tpeeters)]

As requested in this bug https://bugs.launchpad.net/ubuntu-rtm/+source/ubuntu-ui-toolkit/+bug/1513933 I made these changes: https://code.launchpad.net/~tpeeters/ubuntu-ui-toolkit/pageHead-sectionsIndex/+merge/279834 to reset the sectionIndex to -1 when the model changes. The motivation is that the UITK cannot tell which section should be selected after the model changes. The app can tell however, so it can update the selectionIndex after changing the model.