QbsProjectManager: Reparse project before building.

Otherwise, if "save before build" is enabled and the user presses Ctrl+B
with unsaved changes to a project file, these would get ignored due to
the delay (and also if the delay were not there, because the file system
watchers trigger later than the "Build" action).
If there are no actual changes to any build file, the overhead caused by
this operation consists of loading (and possibly storing) the build
graph file.

Task-number: QBS-596
Change-Id: I1f837cc0fcdc77a249b423834f4b6711f5c0bc87
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>