Remove obsolete tooltip database (xml & related class). The tooltip database was only used in one spot. To simplify the code, these tooltips can be defined
directly where they are used.

Some of the tooltips are also present in item classes (and therefore duplicate). This may be worth more refactoring in the future.

mv code from WidgetBox into SampleWidgetBox to remove one unnecessary layer of indirection

=> instead of using the external xml file, fill directly in code

* helps keeping things in sync
* easier to understand
* easier to maintain
* get rid of lots of XML parsing code and error handling

remove also lots of unnecessary code which has apparently been copied from QtDesigner

This removes things which apparently have been copied from Qt internal code (QtDesigner). Contained a huge amount of stuff not used in BornAgain.

this fixes #7 (GUI: After inserting multiple items to the sample editor they should all be selected)

Introduce class ItemWithParticles as base for ParticleItem et. al. Make SessionGraphicsItem virtual base class.

Added some override specifiers in SampeDesigner class definitions and exchanged some 0s in favour of nullptr