UI Fix in TestNG Preferences

I am fixing below two issues in TestNG Plug-in preferences.

1. Currently for Template XML selection, TestNG plug-in display Directory Dialog resulting unable to select a file. Changing this to file dialog with xml filter.
2. In watch testng-results directory dialog, the dialog title says 'Select TestNG Template File" which is not correct. Changing this to "Select TestNG Output Directory".

Please review my change and merge.