Per-app #defines

[CarlosNihelton]

This PR implements a mechanism to let MSBuild set some #defines customised per distro version (i.e. per appx version built).

It relies on the wsl-builder component to apply a file named Preprocessor.props from meta/ subtree according to the appx version being built during the build system preparation, so each Ubuntu app can have its own version of that file with its own list of #defines.

We can later use that mechanism to make MSBuild #define WIN_OOBE_ENABLED for UbuntuPreview and that effectively selects the correct app strategy for the OOBE. The file meta/**/Preprocessor.props must set ItemDefinitionGroup.ClCompile.PreprocessorDefinitions to a list of values that will be passed to the compiler as if the command line /D<VALUE> was passed such as illustrated below:

diff --git a/meta/UbuntuPreview/src/DistroLauncher/Preprocessor.props b/meta/UbuntuPreview/src/DistroLauncher/Preprocessor.props
new file mode 100644
index 0000000..f791092
--- /dev/null
+++ b/meta/UbuntuPreview/src/DistroLauncher/Preprocessor.props
@@ -0,0 +1,8 @@
+<?xml version="1.0" encoding="utf-8"?>
+<Project ToolsVersion="15.0" DefaultTargets="Build" xmlns="https://schemas.microsoft.com/developer/msbuild/2003">
+       <ItemDefinitionGroup>
+               <ClCompile>
+                       <PreprocessorDefinitions>WIN_OOBE_ENABLED</PreprocessorDefinitions>
+               </ClCompile>
+       </ItemDefinitionGroup>
+</Project>

Notice that a default DistroLauncher/Preprocessor.props is not allowed and it's git-ignored. Default #defines must be visible in the source code. That file is for appx-version-specific customisations, thus only those inside meta/ are meant to be version-controlled.

To ensure that the constant definition won't be too early or too late a common Directory.Build.props was placed inside DistroLauncher/ forcing the Preprocessor.props to be imported before common Cpp props (if that file exists, of course).
That required changes in the CI QA workflow, because it was based on placing a custom Directory.Build.props file inside DistroLauncher/ directory to override the ClangTidy MSBuild target. The same behavior can be achieved with a .targets file even though it's loaded later in the build process than the .props file.

This PR just implements the mechanism. The per-app define files must be later created as needed in their respective meta/ subdirectories.