Add properties panel example

[nikku]

Proposed Changes

On top of #32 this adds a properties panel example:

Closes #23

---

Try out via:

cd properties-panel
npm install
npm start

---

Checklist

To ensure you provided everything we need to look at your PR:

• Brief textual description of the changes present
• Visual demo attached
• Steps to try out present, i.e. using the @bpmn-io/sr tool
• Related issue linked via Closes {LINK_TO_ISSUE} or Related to {LINK_TO_ISSUE}