Show attributes dialog with initially unsorted attributes

This commit makes the identify results dialog initially display the attributes unsorted. The user can subsequently sort the attributes by clicking on the respective view headers. The reasoning behind this change was that users might be familiar with the order of attributes in a database table, and expect QGIS to present the attributes in the same order. Possibly one could add a setting to configure the behavior though.

Funded by Sourcepole QGIS Enterprise.