Drag the timeline along, and play through it smoothly

The zoomed track now drags: the handle still drags the time, while the
track around it drags the view along (or the time, with all of it shown),
a tap goes to the time under it, two fingers pinch it, and a sideways swipe
or a shift-wheel drags it too, the same with a mouse, a trackpad or a
finger. A view dragged off the handle stays put until the time moves.

Play moved in steps: the world clock told of its time four times a second,
so the handle and the globe jumped, and play stopped at every weather frame
while it drew, loaded ahead or not. The clock now tells of its time each
animation frame, and play no longer waits on the frame it is at: that
frame loads as play goes on, and play holds only at the next frame while
the one before it still loads. Weather loads its next frame ahead while the
world plays too, tells its listeners of frames as they change rather than
every frame, and the sun and moon move on each minute of world time.

src/i18n/locales/ar.json

@@ -3231,5 +3231,5 @@
   "1 d/s": "يوم/ث",
   "1 wk/s": "أسبوع/ث",
   "Play speed": "سرعة التشغيل",
-  "Scroll or pinch to zoom · double-click to see it all": "مرّر أو اقرص للتكبير · انقر نقرًا مزدوجًا لرؤية الكل"
+  "Scroll or pinch to zoom, drag to move along · double-click to see it all": "مرّر أو اقرص للتكبير، واسحب للتنقل · انقر نقرًا مزدوجًا لرؤية الكل"
 }