clean up the 'qbs' module

the qbs module is a bit of a kitchen sink and therefore contains properties that would better fit into other modules.

the target-related properties pertain to the currently built Product and the toolchain that does it. as such, these properties actually belong into the cpp module (assuming that it serves as a generic toolchain module for native builds, which it currently does). a major complication here is that multiplexing currently works only over properties in 'qbs'; this is covered by QBS-1610.

the host-related properties are fine as-is.

the install-related properties are somewhat questionable to start with; see also QBS-1167.