[FEAT] Consider using the originally planned Loop marker

[tbouffard]

Due to an issue in the proposed loop svg transformation into mxgraph code (see #303 (comment) and #464), we currently (0.1.7-dev) don't use the proposed icon. As we agreed to use this one, give it another chance

Decision

Decided on 2024-04-04 (see #2724)

• reduce width and keep the existing design
• this also require to introduce a mechanism to enforce the marker width: [FEAT] Fix the size of the marker icon #465

[tbouffard]

Be aware of this prototype which doesn't change the loop marker: #2724 (comment)