Remove SessionItem signaling from instruments

Removes usage of SessionItem signaling, replaced by Qt signal/slot. This is part of #88 (closed).

Contains also a few refactorings and simplifications.

To keep current code working, a few oddities are still there, the most notable one is the usage of text-based alignment addressing (programmatically using e.g. "Perpendicular to reflected beam (dpos)" to define which alignment to use). These oddities will be removed in the next MRs when the SessionItem hierarchy system will be removed from instrument items.