[FEAT] Review the message flow start marker

[tbouffard]

The size of the marker may be reviewed (larger?) and the filled color as well.
It is currently transparent so we see the background shape stroke and the flow segment traversing the marker. Having a filled color may probably better highlight the marker.

Proposal with a filled ellipse

See the marker of the top left and bottom right flows.
This has been evoked when implementing the 1st message flow rendering, let's review this now: #394 (comment) and related discussions.

Additional context

The current style configuration explicitly set no fill color.

      FlowKind.MESSAGE_FLOW,
      (style: StyleMap) => {
        style[mxConstants.STYLE_DASHED] = true;
        style[mxConstants.STYLE_DASH_PATTERN] = '8 5';
        style[mxConstants.STYLE_STARTARROW] = mxConstants.ARROW_OVAL;
        style[mxConstants.STYLE_STARTSIZE] = 8;
        style[mxConstants.STYLE_STARTFILL] = false;
        style[mxConstants.STYLE_ENDARROW] = mxConstants.ARROW_BLOCK_THIN;
        style[mxConstants.STYLE_ENDFILL] = false;
      },