[Sandbox::FileAccessor] Add support for private_header_files

Closes #998
[Integration] Update fixtures
Release 0.19.0
Changelog [ci skip]
Changelog [ci skip]

Closes #1012
[Generator::Xcconfig] Add inherited to GCC_PREPROCESSOR_DEFINITIONS

See #903
[TargetInstaller] Create unique hash instances for the build settings

Closes #1014
CHANGELOG [skip ci]
[Integration] Update version in fixtures
Release 0.19.1
[Doc] Make clear that an error occurred in a pre/post hook.
[Rakefile] Perform setup on for integration only if needed.

This skips the unneeded update.
[Rakefile] Don't set the git user as not needed anymore
[Rakefile] Perform setup on for integration only if needed 2
[Cleanup] Move constant closer to code that uses it.
Merge remote-tracking branch 'origin/master' into namespace-xcconfig-linker-flags

Conflicts:
	lib/cocoapods/generator/xcconfig.rb
Update integration fixtures for PODS_LDFLAGS