frontend: Refactored UnitVmModel to support generics

Refactored UnitVmModel to use generics and refactored the code using this
model. To support this the following infrastructure classes has been also
added:
- IntegerEntityModelTextBoxEditor
- IntegerEntityModelTextBoxOnlyEditor

Change-Id: Ie8d177bca97bd0eb31bfcd57d25a481b65485b71
Signed-off-by: Tomas Jelinek <tjelinek@redhat.com>