Used new JSON Form templating option to default tab title to ds name.

The initial value is now {{values.datasources.main[]}}, meaning the
field gets initialized to the name of the datasource. It is mostly
useless when the tab is added from the template configuration page
but that's less likely than automatically adding a datasource for
the time being.

package.json

@@ -141,7 +141,7 @@
                     {
                       "key": "tabs[]",
                       "valueInLegend": true,
-                      "value": "Tab {{idx}}"
+                      "value": "{{values.datasources.main[]}}"
                     },
                     {
                       "title": "Tab data source",