In the "Save Resources" dialog improve usability by better OK button
labeling and use the new ListSelectionDialog API (see bug 573221) for
less code.

If more than one editor is dirty, the "Save Resources" dialog is e.g.
shown when exiting (File > Exit) or when closing all editors
(Ctrl+Shift+W):

In order not to save files/resources, you must first click on "Deselect
All" and then on "Save Selected". It feels wrong to me when I have to
click "Save Selected" for don't save.

For this reason change the OK button label to "Don't Save" when no
resource is selected, otherwise to "Save <n> of <m>" (<n> number of
selected resources; <m> total number of resources).

Change-Id: Ie8a661c2a4ee72f9031042a84380e04ffd056669
Reviewed-on: https://git.eclipse.org/r/c/platform/eclipse.platform.ui/+/179468
Tested-by: Platform Bot <platform-bot@eclipse.org>
Reviewed-by: Lars Vogel <Lars.Vogel@vogella.com>