Online repositories: Add possibility to define base path in metadata for payload

Creating online repositories or offline installers take long time as the packaging needs to:

1) extract build artifacts
2) prepend the rest of the installation path, e.g. "5.6/android_armv7"
3) re-create the payload archive per installer component

Especially on windows this takes hours per Qt snapshot.

We could eliminate these steps completely by defining additional metadata per installer component.

Example:

qt.56.win32_msvc2013

• package.xml:
  <TargetDir>5.6/win32_msvc2013</TargetDir>

Now when the installer extracts the payload it can add this <TargetDir> into the path if defined (backwards compatible change).

This would speed a lot creating installers/online repositories.