gtk4-prep: restore lighttable scrolling over thumbnail overlays + better smooth scrolling

[Arecsu]

Fixes #21735 — scrolling the lighttable stopped as soon as the pointer was over one of the always-visible thumbnail overlay icons (e.g. the star ratings). Scrolling worked everywhere else: over the thumbnails themselves, and with "overlays on hover" mode.

This was a regression from the scroll-event → event-controller conversion (#21659), which broke two things in the lighttable and culling views:

• Scrolling over the overlay icons stopped working. The new scroll handlers were attached in "target-only" mode, so they only ran when the pointer was over the view's own background — not over a child widget such as an always-visible rating icon. The old signal-based code received those scrolls via bubbling, which we've restored.
• Touchpad scrolling became sluggish and jumpy. Smooth touchpad input was being scaled down and chopped into fixed 50px steps. The grid now reads the raw scroll deltas again (as it did before the conversion), so scrolling is continuous and follows the finger 1:1.

No behavior changes for mouse wheels, and nothing new is turned on by default — this restores how the lighttable scrolled before the conversion.

Related: #21735 #15920 #20433

[Arecsu]

Superseded by #21745