Unsaved changed are lost after opening new project

[jozef-budac]

When there are unsaved changes (like new features) in current project and "Open QGIS project" is used to open different project, all changes are lost. It should be good to check and notify user, if there are unsaved changes, that can be lost.

[mergin-maps-bot]

Users report data lost when they open MM projects from Browser panel. The MM plugin should warn them if there are any unsaved changes (either to project or layers).