Generate projects for .NET 4.7 to use new features

An example is LPUTF8Str which can be used for marshalling UTF-8 strings.

Signed-off-by: Dimitar Dobrev <dpldobrev@protonmail.com>

build/Helpers.lua

@@ -138,7 +138,7 @@ function SetupManagedProject()
   location ("%{wks.location}/projects")
   buildoptions {"/platform:".._OPTIONS["arch"]}
 
-  dotnetframework "4.6"
+  dotnetframework "4.7"
 
   if not os.istarget("macosx") then
     filter { "action:vs* or netcore" }