Draw guides for line handles

[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?

When moving a line end (a handle), no guides are created to help align the line end.

Issue Number: gaphor/gaphor#1006

What is the new behavior?

Handles are aligned with element. Line handles are also aligned with their own handles, so it's easier to make a line horizontal or vertical, without enabling the "orthogonal" setting.

Does this PR introduce a breaking change?

• Yes
• No

Other information

• All Guide behavior has moved to mixins.
• There's no longer need for a HandleMove mixin for segments: this is all handled by the tool.

[sourcery-ai]

Sourcery Code Quality Report

✅  Merging this PR will increase code quality in the affected files by 0.36%.

Quality metrics	Before	After	Change
Complexity	3.15 ⭐	2.54 ⭐	-0.61 👍
Method Length	42.38 ⭐	44.04 ⭐	1.66 👎
Working memory	7.10 🙂	7.10 🙂	0.00
Quality	75.65% ⭐	76.01% ⭐	0.36% 👍

Other metrics	Before	After	Change
Lines	743	772	29

Changed files	Quality Before	Quality After	Quality Change
gaphas/guide.py	79.01% ⭐	81.76% ⭐	2.75% 👍
gaphas/matrix.py	89.83% ⭐	89.83% ⭐	0.00%
gaphas/segment.py	68.34% 🙂	67.42% 🙂	-0.92% 👎
tests/test_guide.py	76.15% ⭐	75.71% ⭐	-0.44% 👎

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

File	Function	Complexity	Length	Working Memory	Quality	Recommendation
gaphas/segment.py	LineSegment.merge_segment	9 🙂	165 😞	10 😞	52.62% 🙂	Try splitting into smaller methods. Extract out complex expressions
gaphas/segment.py	LineSegment.split_segment	6 ⭐	202 😞	9 🙂	54.38% 🙂	Try splitting into smaller methods
gaphas/segment.py	LineSegmentPainter.paint	4 ⭐	144 😞	12 😞	55.96% 🙂	Try splitting into smaller methods. Extract out complex expressions
gaphas/matrix.py	Matrix.set	5 ⭐	78 🙂	12 😞	63.56% 🙂	Extract out complex expressions
gaphas/segment.py	LineSegment.split	4 ⭐	90 🙂	11 😞	64.52% 🙂	Extract out complex expressions

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!