Merge fly view and joystick custom actions in a single backend

[DonLakeFlyer]

Currently the Fly View and Joysticks have support for defining and executing arbitrary mavlink commands. Right now those are two different systems which work differently and use different file formats/location. This pull combines the two into a single backend which works the same way for both.

Changes:

• Custom action json files are always loaded from the CustomActions directory of the normal QGC save path.
• The Fly View loads the file named FlyViewCustomActions.json
• The Joysticks load the file named JoystickCustomActions.json
• None of the above things are configurable
• The existence of the files are optional
• Updated the user guide docs to explain how it works as well as the file format for the json file

Change in function from before:

• There is no longer a setting to turn off Fly View custom actions. If you don't want them don't put the file there.
• There is no longer a setting to specify the Fly View custom actions file
• The thinking is that was unnecessary additional complexity which doesn't add much benefit

ToDo:

• Warn the user who was doing it the old way that it has changed and they will need to update their files/locations.

Related to #11205

[DonLakeFlyer]

@nexton-winjeel FYI: As to whether to change to not allow filename setting is a problem?

[nexton-winjeel]

FYI: As to whether to change to not allow filename setting is a problem?

@DonLakeFlyer: It won't break anything, but it will be an annoyance. The reason we have the ability to select different files from the UI was so that we can use the one GCS setup for multiple vehicle configs, and quickly swap between custom actions if we change the active vehicle.

[DonLakeFlyer]

Hmm, that's a reasonable use case. I'll put back the ability to switch files.

[DonLakeFlyer]

Updates:

• You can pick the Fly View and Joystick action file from Fly View Settings
• User is warned about needing to redo their custom actions settings

[DonLakeFlyer]

@nexton-winjeel Can you help with testing this?

[nexton-winjeel]

@nexton-winjeel Can you help with testing this?

@DonLakeFlyer: I pulled the Linux AppImage from https://github.com/mavlink/qgroundcontrol/actions/runs/8197777789/artifacts/1308087842. Confirmed:

• can load and use custom JSON for Fly View;
• can load and use custom JSON for Joystick;
• can change custom JSON at runtime for Fly View; and
• can change custom JSON at runtime for Joystick.

One thing I did note: The description field is now required, whereas previously it was optional. The description field isn't used by the Joystick side, so may force users to enter an unused field in their custom files.

[DonLakeFlyer]

The description field isn't used by the Joystick side, so may force users to enter an unused field in their custom files.

Yeah, that's slightly odd. But I wanted the files to be shareable between joystick and fly. Seems useful to just use a single file for both.

[ivanzivotic]

@DonLakeFlyer thanks for this update. Just a note, I think there is a mistake in the docs/en/SUMMARY.md path, causing the docs website to break when trying to open the custom actions tab. Namely, there is an extra '(' at the link start and missing 's' in the filename before the .md extension

[DonLakeFlyer]

@ivanzivotic Thanks for letting me know: #11246