Qt4BC: Simplify shadowbuild/builddirectory handling

Change-Id: Id46a239588d21b6aa5d4c43e4d85b52a04bd17ab
Reviewed-by: Daniel Teske <daniel.teske@digia.com>
BC: Move builddirectory handling into BC itself

Use setBuildDirectory() in the different BuildConfigurations instead
of reimplementing that over and over again.

Change-Id: Ic355fdb4624c71667ce470b3e2865c9a8722ef09
Reviewed-by: Daniel Teske <daniel.teske@digia.com>
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>