chore: make properties panel a keyboard trap

This ensure properties panel focus is not lost when I mouse down on,
i.e. a label. On the other hand this remains unfocusable through
keyboard navigation. We can consider to change that some day, just
not today 😉.

src/render/BpmnPropertiesPanelRenderer.js

@@ -44,7 +44,7 @@ export default class BpmnPropertiesPanelRenderer {
     this._getFeelPopupLinks = getFeelPopupLinks;
 
     this._container = domify(
-      '<div style="height: 100%" class="bio-properties-panel-container"></div>'
+      '<div style="height: 100%" tabindex="-1" class="bio-properties-panel-container"></div>'
     );
 
     domEvent.bind(this._container, 'focusin', (event) => this._checkFocus(event));