Fix text selection issues in chrome (#425)

Chrome's default selection behavior when pressing the shift key
messes with styles/selection hightlighting in the diagram

By setting user-select:none for the diagram svg we can fix this issue.
Default html text selection does not work for svg text elements anyways,
so this change does not break any existing behavior.

Author: tortmayr

packages/client/css/glsp-sprotty.css

@@ -247,6 +247,7 @@
     border-style: solid;
     border-width: 1px;
     border-color: #bbb;
+    user-select: none;
 }
 
 .sprotty text {