Fix splitting of lines

[amolenaar]

PR Type

What kind of change does this PR introduce?

• Bug fix
• Feature
• Chore (refactoring, formatting, local variables, other cleanup)
• Documentation content changes

What is the current behavior?

It's no possble to split a line in two segments.

Issue Number: #1312

What is the new behavior?

Start a transaction on drag-begin, So handle creation happens within a transaction.

Reverts d555859 (except for one line).

Fixes #1312.

Does this PR introduce a breaking change?

• Yes
• No

[sourcery-ai]

Sourcery Code Quality Report

✅  Merging this PR will increase code quality in the affected files by 0.28%.

Quality metrics	Before	After	Change
Complexity	1.22 ⭐	1.09 ⭐	-0.13 👍
Method Length	22.50 ⭐	21.50 ⭐	-1.00 👍
Working memory	4.98 ⭐	4.89 ⭐	-0.09 👍
Quality	88.63% ⭐	88.91% ⭐	0.28% 👍

Other metrics	Before	After	Change
Lines	70	63	-7

Changed files	Quality Before	Quality After	Quality Change
gaphor/diagram/tools/txtool.py	96.23% ⭐	97.42% ⭐	1.19% 👍
gaphor/diagram/tools/tests/test_txtool.py	82.24% ⭐	82.44% ⭐	0.20% 👍

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

File	Function	Complexity	Length	Working Memory	Quality	Recommendation

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.

---

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!