Split effects into image and video effects, with VHS tracking as the first video effect

Effect::video() says which kind an effect is. The Add effect menu lists the
two kinds in sections of their own, each scrolling on its own, and offers
video effects only in a video project; a video stage carries a film icon in
the chain and, in a still, is dimmed and skipped like a stage switched off.
enabledStages takes whether the render is a video and is the one place that
leaves video stages out of a still. --help groups the video effects under a
heading of their own, --list-effects marks them, and a video effect given
for a still is skipped with a warning.

VideoEffect is the base every video effect builds on, and keeps the rules a
moving stage has to follow: it takes one value from the shared rng, the same
on every frame, and hands the effect steady and per-frame generators and
loop-rounded time through a Moment, so a subclass writes only animate().
VHS tracking is the first, and the one to copy: the colour planes sway and
shake and a tracking bar rolls down the picture.

That motion moves out of channel split, which is an image effect again and
renders stills exactly as before; forVideo() goes with it. Version 0.7.0.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/core/params.cpp

@@ -16,12 +16,6 @@ ParamSpec ParamSpec::styled() const {
     return copy;
 }
 
-ParamSpec ParamSpec::forVideo() const {
-    ParamSpec copy = *this;
-    copy.videoOnly = true;
-    return copy;
-}
-
 ParamSpec ParamSpec::makeInt(std::string key, std::string label, long defaultValue, long minimum,
                              long maximum, std::string help, long step) {
     ParamSpec spec;