[sld-viewer] Center navigation arrow

[flo-dup]

Please check if the PR fulfills these requirements

• [x] The commit message follows our guidelines
• [ ] Tests for the changes have been added (for bug fixes / features)
• [ ] Docs have been added / updated (for bug fixes / features)

Does this PR already have an issue describing the problem? No

What kind of change does this PR introduce? Bug fix

What is the current behavior? Navigation arrow is not centred in the cell except for lines.

What is the new behavior (if this is a feature change)? Navigation arrow centred for two-winding transformers and HVDC.

Does this PR introduce a breaking change or deprecate an API?

• [ ] Yes
• [x] No

[flo-dup]

Tested thanks to the demo: works on phase-shift transformers, two-windings transformers, LCC, VSC (and still works on lines)