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).

Principia/tools/tools.vcxproj

Line 18 in 874e4db

<ClCompile Include="..\base\cpuid.cpp" />

Principia/journal/journal.vcxproj

Line 29 in 9b518f6

<ClCompile Include="..\base\cpuid.cpp" />

Principia/quantities/quantities.vcxproj

Line 38 in 9b518f6

<ClCompile Include="..\base\cpuid.cpp" />

Principia/mathematica/mathematica.vcxproj

Line 19 in bc6b5ef

<ClCompile Include="..\base\cpuid.cpp" />

Principia/benchmarks/benchmarks.vcxproj

Line 19 in 3f99fea

<ClCompile Include="..\base\cpuid.cpp" />

Principia/integrators/integrators.vcxproj

Line 40 in bc6b5ef

<ClCompile Include="..\base\cpuid.cpp" />

Principia/testing_utilities/testing_utilities.vcxproj

Line 51 in 13bb9cc

<ClCompile Include="..\base\cpuid.cpp" />

Principia/geometry/geometry.vcxproj

Line 63 in bc6b5ef

<ClCompile Include="..\base\cpuid.cpp" />

Principia/ksp_plugin_test/ksp_plugin_test.vcxproj

Line 19 in aa753cb

<ClCompile Include="..\base\cpuid.cpp" />

Principia/ksp_plugin/ksp_plugin.vcxproj

Line 65 in 3f99fea

<ClCompile Include="..\base\cpuid.cpp" />

Principia/numerics/numerics.vcxproj

Line 83 in a923fd4

<ClCompile Include="..\base\cpuid.cpp" />

Principia/physics/physics.vcxproj

Line 87 in e27713c

<ClCompile Include="..\base\cpuid.cpp" />

Principia/astronomy/astronomy.vcxproj

Line 107 in 07e8049

<ClCompile Include="..\base\cpuid.cpp" />