Allow adding the same project in different locations

[Guts]

Feature description

For now, it's 1 line = 1 project = 1 location. An end-user might want to generate a menu in different places from the same project.

Identified steps:

• Use QgsCheckableComboBox instead of QComboBox
• Adapt the configuration widget to switch from a dropdown menu to non-exclusive checkboxes.
• Add the related logic
• handle legacy configurations
• make sure the cache is used only once for the same project generation

Additional context

This feature request is formulated from Agences de l'Eau and ported by Oslandia.