Enhance the identify on WMS using GetFeatureInfo

[jgrocha]

Author Name: aperi2007 - (aperi2007 -)
Original Redmine Issue: 5892

Redmine category:web_services_clients/wms

---

(see also ticket #15340)
Actually qgis always call an identify on WMS using
INFO_FORMAT=text/plain
An useful enhanced should be add a properties at wms-layer-level settable from user to choose the format for the getfeatureinfo request.
The available values should be:

text/plain
text/xml
text/html

---

Related issue(s): #15340 (duplicates), #15770 (relates)
Redmine related issue(s): 5887, 6575

---