Fix Ctrl-scroll zoom for GTK4

[amolenaar]

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Documentation content changes

What is the current behavior?

Apps like Firefox and Inkscape use Ctrl+scroll wheel to zoom.

Issue Number: gaphor/gaphor#1798

What is the new behavior?

Scroll-zoom works on GTK4.
For GTK3 it's not possible, since scroll events can't be propagated.

[sourcery-ai]

Sourcery Code Quality Report

✅  Merging this PR will increase code quality in the affected files by 0.92%.

Quality metrics	Before	After	Change
Complexity	5.82 ⭐	5.57 ⭐	-0.25 👍
Method Length	85.69 🙂	83.34 🙂	-2.35 👍
Working memory	2.66 ⭐	3.09 ⭐	0.43 👎
Quality	60.35% 🙂	61.27% 🙂	0.92% 👍

Other metrics	Before	After	Change
Lines	471	521	50

Changed files	Quality Before	Quality After	Quality Change
examples/demo.py	57.16% 🙂	57.28% 🙂	0.12% 👍
gaphas/tool/init.py	%	%	%
gaphas/tool/zoom.py	80.78% ⭐	78.00% ⭐	-2.78% 👎

Here are some functions in these files that still need a tune-up:

File	Function	Complexity	Length	Working Memory	Quality	Recommendation
examples/demo.py	create_window	13 🙂	851 ⛔		31.45% 😞	Try splitting into smaller methods
examples/demo.py	create_canvas	1 ⭐	267 ⛔	6 ⭐	62.56% 🙂	Try splitting into smaller methods
gaphas/tool/zoom.py	on_scroll	2 ⭐	101 🙂	10 😞	66.87% 🙂	Extract out complex expressions

Legend and Explanation

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• 🙂 good
• 😞 poor
• ⛔ very poor

The 👍 and 👎 indicate whether the quality has improved or gotten worse with this pull request.

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