When GLArea gets a GL error it leaves user with nothing, but transparent background with error text in the middle. Make it look somewhat decent by drawing black background for the not working video widget.
The button next to the currently playing playlist item acts as a close window button in order to allow closing app without leaving fullscreen. It should use the theme "window-close" icon to better show what it does.
In case of local files the "filename" variable holds the string that is later assigned to the filename property (no difference which is used), but when playing URIs only filename property holds the string. Passing an undefined value to the label caused an error in this case and prevented playback of URIs.
Ship custom gtk4glsink plugin as part of API insead of normal gstreamer plugin. This avoids gstreamer plugin registry conflicts with gtk3 plugin and allows more customization.
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.
Default "seek" function takes time in nanoseconds as argument which is not that useful, cause we will start playback from the nearest keyframe anyway. The new "seek_seconds" can take a double value for more precise seeking.
