Fix library clean/scan after #5324 when started from a remote.

PR #5324 hides progress bars (and other dialogs in general) when a library clean
or scan is started from a built-in function ("CleanLibrary", "UpdateLibrary").
This is fine as it was assumed that built-ins are only executed by remote
clients and/or add-ons, but built-ins can be mapped to remote or keyboard
buttons.

This adds an "isRemote" boolean to CBuiltins::Execute() to check if the action
was executed from a remote or a keyboard.

I do believe this is the cleanest solution and can be reused in the future if needed.

See: http://forum.kodi.tv/showthread.php?tid=207697