Use shared items projects

[eggrobin]

This is a mess already, see below. With modules we would need to do that for every file. We should instead put the files currently under the filters Header Files and Source Files for each project into a shared items project, depended upon by whichever project needs them (including the test projects that currently contain them).

https://github.com/mockingbirdnest/Principia/blob/874e4dbc85853a57ca3de63b7d93b45794860be2/tools/tools.vcxproj#L18 https://github.com/mockingbirdnest/Principia/blob/9b518f6181d5f9d0a57d4463d9c824ffe5700006/journal/journal.vcxproj#L29 https://github.com/mockingbirdnest/Principia/blob/9b518f6181d5f9d0a57d4463d9c824ffe5700006/quantities/quantities.vcxproj#L38 https://github.com/mockingbirdnest/Principia/blob/bc6b5efff855abefad8af36eacb40126babcc800/mathematica/mathematica.vcxproj#L19 https://github.com/mockingbirdnest/Principia/blob/3f99fea62d2fcf49f65bb0ce075b25c19db68413/benchmarks/benchmarks.vcxproj#L19 https://github.com/mockingbirdnest/Principia/blob/bc6b5efff855abefad8af36eacb40126babcc800/integrators/integrators.vcxproj#L40 https://github.com/mockingbirdnest/Principia/blob/13bb9cccb5a477e78061acfebd7526e3e5f91ddf/testing_utilities/testing_utilities.vcxproj#L51 https://github.com/mockingbirdnest/Principia/blob/bc6b5efff855abefad8af36eacb40126babcc800/geometry/geometry.vcxproj#L63 https://github.com/mockingbirdnest/Principia/blob/aa753cba8200fece87ceeec4f3b165b469aac41a/ksp_plugin_test/ksp_plugin_test.vcxproj#L19 https://github.com/mockingbirdnest/Principia/blob/3f99fea62d2fcf49f65bb0ce075b25c19db68413/ksp_plugin/ksp_plugin.vcxproj#L65 https://github.com/mockingbirdnest/Principia/blob/a923fd4378f47dcd5f22a1524f52909100362da2/numerics/numerics.vcxproj#L83 https://github.com/mockingbirdnest/Principia/blob/e27713ccfce53a9ea6238e58782439a1043038fb/physics/physics.vcxproj#L87 https://github.com/mockingbirdnest/Principia/blob/07e80491344b03376f4d2dc5f5e581592b3dd1b3/astronomy/astronomy.vcxproj#L107