Support past Revit versions

[RenatoCaenaro]

Why do not have multiple project for each version of Revit in the solution?

[TrabacchinLuigi]

I'm trying to take advantage of the new csproj format to create a shared codebase and a project for each version we want to support, instead of having a branch per version.
This way we could also insert condirtional compilation symbols like revit2021 or revit2020 in the projects.
But i'm facing some trubles since resources and graphics aren't properly separated from code.