GUI: RealModel is refactored

RealModel dependence from SessionModel was significantly reduced.

Real items were disconnected from RealModel's "root_item", they are stored in the vector now, not tree.

Read/write system of RealModel was simplified and made more explicit.

XML tags were localized: they are defined now in the same file with read/write methods.

RealModel and JobModel have much in common, so they should inherit some base class that will replace SessionModel. This is not done yet.