Implement in-scene popup overlay system in FXMLMainMenu and wire secondary views via FXMLLoader#1

Dsannue with Copilot wants to merge 2 commits into

masterDsannue/PracticaIPC:masterfrom

copilot/combine-xml-popupsDsannue/PracticaIPC:copilot/combine-xml-popupsCopy head branch name to clipboard