Reset external application choices

Settings screen (#77) should have option to reset all choices made in app chooser during external calls.
#166 will contain similar options on _per-action_ basis, but might long time pass till we implement it.
This issue suggests quicker solution.

Start by reviewing the Settings screen described in issue #77 and the per-action reset context in issue #166. Identify where choices made in the app chooser for external calls are stored and exposed in Settings; done means a single option resets all such choices.

Written by the indexing model from the issue text.