create tag with product value set

Addresses issue #125.

I made two tests that are similar but yet different. 
One test simply selects product and creates tag, another one not only selects product, but also includes caseversions. I can remove on of them or we can have them both. There is no mention in github issue what actually test should check.

While working on that issue, I recognized that multiselect widget is used across several PageObjects, so I moved all   related logic into separate PageRegion object and did some refactoring.

Another note: it appears that editing tag with some product set causes 500 server error in ajax request on edit tag page and thus no caseversions are loaded. I checked in bugzilla and saw one related bug, but maybe someone from the team needs to look at that issue more closely.