API: Expose the target directory for an installable file.

This is the information that is provided in a qbs project file, and we
should make it easily available via the API. Providing the full file
path is nice, but users must not be forced to manually "subtract" the
file name to get at the directory.

Change-Id: I0368013fca9c7cad86778fa46fdc6a6352bd8472
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>
gcc: use an absolute path for the precompiled header include

Task-number: QBS-378

Change-Id: I024ab50578a02e7c48c8488becabdd2fe003d4f6
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>
add autotest for precompiled headers

Change-Id: Idf4a34615541a2ec187745d6195ecdbd34f51a8e
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>