Add zoom using Ctrl+scroll

[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?

Zoom with mouse is not really possible.

Issue Number: gaphor/gaphor#877

What is the new behavior?

Zoom works with Ctrl+scroll

Does this PR introduce a breaking change?

• Yes
• No

Other information

Added:

• Ctrl + middle mouse button scroll: zoom in
• Middle mouse button press + move: pan canvas

[sourcery-ai]

Sourcery Code Quality Report

✅  Merging this PR will increase code quality in the affected files by 0.09%.

Quality metrics	Before	After	Change
Complexity	1.58 ⭐	1.65 ⭐	0.07 👎
Method Length	51.33 ⭐	49.39 ⭐	-1.94 👍
Working memory	6.03 ⭐	6.21 🙂	0.18 👎
Quality	73.66% 🙂	73.75% 🙂	0.09% 👍

Other metrics	Before	After	Change
Lines	1095	1147	52

Changed files	Quality Before	Quality After	Quality Change
examples/demo.py	70.67% 🙂	70.60% 🙂	-0.07% 👎
examples/demo4.py	71.36% 🙂	71.35% 🙂	-0.01% 👎
gaphas/tool/init.py	%	%	%
gaphas/tool/scroll.py	85.70% ⭐	74.18% 🙂	-11.52% 👎
gaphas/tool/zoom.py	78.75% ⭐	82.51% ⭐	3.76% 👍
tests/test_tool_zoom.py	92.64% ⭐	92.59% ⭐	-0.05% 👎

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	4 ⭐	655 ⛔	9 🙂	47.92% 😞	Try splitting into smaller methods
examples/demo4.py	create_window	4 ⭐	640 ⛔	9 🙂	47.93% 😞	Try splitting into smaller methods
gaphas/tool/scroll.py	on_scroll	6 ⭐	126 😞	15 😞	51.83% 🙂	Try splitting into smaller methods. Extract out complex expressions
examples/demo.py	create_canvas	1 ⭐	221 ⛔	6 ⭐	65.03% 🙂	Try splitting into smaller methods
examples/demo4.py	create_canvas	1 ⭐	221 ⛔	6 ⭐	65.03% 🙂	Try splitting into smaller methods

Legend and Explanation

The emojis denote the absolute quality of the code:

• ⭐ excellent
• 🙂 good
• 😞 poor
• ⛔ very poor

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

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Please see our documentation here for details on how these metrics are calculated.

We are actively working on this report - lots more documentation and extra metrics to come!

Help us improve this quality report!

[danyeaw]

This looks great, nice feature to add 👍