Reorg dirs: disentangle Models from Views

See merge request !93

Refactor model: instrument, footprint and background items

See merge request !92

Introduce SessionItem::setGroupPropertyType, a templated version of SessionItem::setGroupProperty, to reduce the need for casting

Reorg dirs: reorganise `MaterilaItemUtils`

See merge request !91

Reorg dirs: Move Error class from utils/GUIHelpers to Models/Error

See merge request !90

Remove RegionOfInterest

See merge request !88