[subtitle] load .sup file like other external subtitles.

[taxigps]

Description

Scan .sup file use ScanForExternalSubtitles and support to load .sup file in GUIDialogSubtitleSettings

Motivation and context

https://forum.kodi.tv/showthread.php?tid=356403

How has this been tested?

What is the effect on users?

Screenshots (if appropriate):

Types of change

• Bug fix (non-breaking change which fixes an issue)
• Clean up (non-breaking change which removes non-working, unmaintained functionality)
• Improvement (non-breaking change which improves existing functionality)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that will cause existing functionality to change)
• Cosmetic change (non-breaking change that doesn't touch code)
• None of the above (please explain below)

Checklist:

• My code follows the Code Guidelines of this project
• My change requires a change to the documentation, either Doxygen or wiki
• I have updated the documentation accordingly
• I have read the Contributing document
• I have added tests to cover my change
• All new and existing tests passed

[taxigps]

@enen92 reworked. please review again.

[CastagnaIT]

i think you have to add the subtitle extension also in to these files (look the lists):
xbmc/network/upnp/UPnPInternal.cpp
xbmc/platform/darwin/ios/Info.plist.in

[taxigps]

@CastagnaIT updated

[enen92]

Lgtm, only suggestion is to make -1 some error constexpr so it conveys something meaningful but it's fine either way.