Describe the bug
A BPMN diagram containing a pool or a lane requests unavailable images (this generates http 404 errors)

To Reproduce
Seen with 0.1.1 and 0.1.2:

1. Open the A.4.0.bpmn file from https://github.com/bpmn-miwg/bpmn-miwg-test-suite Reference
2. The images/expanded.gif resources is requested but not available

Expected behavior
No 404 error: the image is displayed or not requested if not needed

Additional context
We do not provide expand/collapse capabilities and this image could be used to provide a link for such a purpose, so it seems not needed

mxGraph information

• folding (collapse and expand via an image icon in the graph) is enabled by default: see https://github.com/jgraph/mxgraph2/blob/fcaf6cc2a733ee250795277af9f18ab0dd73aa02/javascript/src/js/view/mxGraph.js#L1122
• the folding image url is computed by https://github.com/jgraph/mxgraph2/blob/mxgraph-4_1_1/javascript/src/js/view/mxGraph.js#L8838
• the image path is defined in https://github.com/jgraph/mxgraph2/blob/fcaf6cc2a733ee250795277af9f18ab0dd73aa02/javascript/src/js/view/mxGraph.js#L1609
• the image path can be configured: https://jgraph.github.io/mxgraph/docs/js-api/files/index-txt.html: “Built-in Images” section
• could we provide/define the image directly in the code by encoding the image using base64 like in https://github.com/jgraph/mxgraph2/blob/dad6e78477c7a30f9701dee49b5a65555e296415/javascript/examples/grapheditor/www/js/Sidebar.js#L108