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

* refactor IDetector to have built-in, n-dim ROI support
* add methods for implicit ROI handling (no distinction to be made on caller's side)
* add docu for ROI
* refactor CoordSystem classes to not use detector and its ROI related things internally,
  but at caller's side. This eases transition handled in issue #71 ("Further consolidate ICoordSystem")

Refactor model: limits items

See merge request !87

Moved ExternalProperty files to Models

See merge request !86

Refactor Model: distribution items

See merge request !85

Redesign mask editor so it shows already while editing what you will get (WYSIWYG)

See merge request !84

fix project reading error introduced with global renaming of "OffSpecInstrument"

See merge request !83

This removes the necessity of ROI to implement the "show" button, to see the masking effects.

In a further step the mask presenter (which uses ROI functionality) could be removed, which was the motivation for the refactoring/redesign in the context of "remove ROI".

fix project reading error introduced with global renaming "OffSpecInstrument" -> "OffSpecularInstrument"

Make BeamDistributionItem::setDistributionGroupItem a template to reduce the need for casting and improve type safety

Refactor model: SymmetricDistributionItem

See merge request !82

Refactorings no. 2 for the upcoming removing of "region of interest" (ROI)

See merge request !81