Custom Visual Studio project generation logic that supports any platform that has a msvs.py
script, so Visual Studio can be used to run scons for any platform, with the right defines per target.
Invoked with `scons vsproj=yes`
To generate build configuration files for all platforms+targets+arch combinations, users should call
```
scons vsproj=yes platform=XXX target=YYY [other build flags]
```
for each combination of platform+target[+arch]. This will generate the relevant vs project files but
skip the build process, so that project files can be quickly generated without waiting for a command line
build. This lets project files be quickly generated even if there are build errors.
All possible combinations of platform+target are created in the solution file by default, but they
won't do anything until each one is set up with a scons vsproj=yes command for the respective platform
in the appropriate command line. This lets users only generate the combinations they need, and VS
won't have to parse settings for other combos.
Only platforms that opt in to vs proj generation by having a msvs.py file in the platform folder are included.
Platforms with a msvs.py file will be added to the solution, but only the current active platform+target+arch
will have a build configuration generated, because we only know what the right defines/includes/flags/etc are
on the active build target currently being processed by scons.
Platforms that don't support an editor target will have a dummy editor target that won't do anything on build,
but will have the files and configuration for the windows editor target.
To generate AND build from the command line, run
```
scons vsproj=yes vsproj_gen_only=no
```
Andreia Gaita