Step 5 of the gui/app.py + gui/preview.py structural refactor. Moves
the click-to-seek QSlider variant and the mpv-backed transport-control
widget into their own module under media/. The new module imports
_MpvGLWidget from .mpv_gl (sibling) instead of relying on the bare
name in the old preview.py namespace.
Address-only adjustment: the lazy `from ..core.cache import _referer_for`
inside `play_file` becomes `from ...core.cache import _referer_for`
because the new module sits one package level deeper. Same target
module, different relative-import depth — no behavior change.
preview.py grows another re-export shim line so ImagePreview (still
in preview.py) and FullscreenPreview can keep constructing
VideoPlayer unchanged. Shim removed in commit 14.
