Otherwise GStreamer will error out and playback will stop. Non-existing subtitle file is common on Flatpak when user tries to open/drop a file from a folder that container does not have access to.
Play video with external subtiles by:
* selecting and opening both video+subs from file chooser/manager
* dropping subtitles file on player window
* opening subtiles from file chooser/manager while video plays
* send their file path/uri to player via WebSocket message
The "clapper_src" directory name was unusual. This was done to make it work as a widget for other apps. Now that this functionality got removed it can be named simply "src" as recommended by guidelines.
