[FIX] sap.m.Panel: Tabindex is now added only in FF
[FIX] sap.m.Panel: Tabindex is now added only in FF

The tabindex attribute is needed only in FF for fixing a browser dependant issue, which adds an additional tab stop. Otherwise, it should not be present, because it exposes the html element in the accessible tree and triggers unwanted speech output.

Change-Id: Ie9818821da0538abb361778d1c04695bdb2b3cf1
BCP: 1870031994

cschuff/openui5,SAP/openui5,cschuff/openui5,SAP/openui5,SAP/openui5,SAP/openui5,cschuff/openui5