[Qt Frontend] Improve architecture

[sgsaenger]

The current architecture of the main Vipster frontend (vipster/gui/qt) is too convoluted and not maintainable.
Development of a new application architecture is performed on a branch https://github.com/sgsaenger/vipster/tree/architecture.

Acceptance criteria

• All application state is handled via a separate class instance (Vipster::Application)
  • GUI classes and widgets should store only state required for rendering
  • All data (potentially) shared between widgets is stored in the application
• event handling is done via Qt signals/slots
  • the BaseWidget class is removed
  • other implementations of triggerUpdate/updateWidget are removed
• State is minimized during the conversion process, e.g.:
  • raw pointers should be removed in general (if possible)
  • widgets should access molecule data mostly (or only?) in the event handling process
• Investigate off-loading time-consuming operations to threads