QbsProjectManager: Support using an installed qbs.

Distributions will want to make Creator work with
their qbs package instead of adding it a second time
via the submodule, so we support that by checking for
the QBS_INSTALL_DIR environment variable.
This supersedes the current mechanism using QBS_SOURCE_DIR
and QBS_BUILD_DIR.

Change-Id: I2cff651cd23ea47b1a1434e33f80cb74ca0fc96b
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>