QbsPM: Add project's build options widget

Would be good idea to integrate into QbsPM a widget for display and editing of project properties in user-friendly form.

For example, for me as user, it would be much more pleasant to edit optimization parameters, flags of the compiler/linker, linked libraries, include paths and so forth in convenient UI, instead of project file *.qbs.

Many IDE (for example VisualStudio, Code::Blocks and others) give such opportunity.

I would suggest to place this widget on "Build Settinds" page, approximately as here: http://postimg.org/image/p157151fl/

This idea was discussed in the mailing list in a subject "Share some classes of QBS project manager plugin.", e.g.:

http://www.marshut.com/spimt/share-some-classes-of-qbs-project-manager-plugin.html