Expose full set of script execution options in settings window

Summary:
Right now the script execution setting is a checkbox, capable only of handling boolean
input. This is not appropriate because there are actually three options available.

Accordingly, this patch replaces the checkbox with a combobox (to save space compared
to radio buttons) that clearly exposes all three options.

BUG: 371837
FIXED-IN: 19.12.0

Test Plan:
{F7440559}

- All three options do what they say they'll do
- All options get saved properly
- The combobox displays the correct option when the window is loaded
- The {nav Defaults} button resets it to "always ask" as expected

Reviewers: elvisangelaccio, #dolphin, #vdg

Reviewed By: elvisangelaccio, #dolphin

Subscribers: kfm-devel

Tags: #dolphin

Maniphest Tasks: T9932

Differential Revision: https://phabricator.kde.org/D24247