Improve UI of view, groupBy and visualization option selection for report creation

AT:

• The select components of the report control panel are replaced with dropdown components
• The view dropdown contains submenus for Frequency Count, Process Instance Duration and Flownode Duration
• The group by dropdown contains submenus for Start Date and Variables
• The Variables submenu follows the specification found at https://zpl.io/Vk51n08
• It is still impossible to create invalid combinations, by disabling certain dropdown entries and providing an autocomplete wizard for unambiguous choices