Add SortFilterProxyModel.qbs for inclusion into Qbs projects

[mitchcurtis]

This can be used similary to how the .pri is used. For example, to build it as part of an existing library:

import "../3rdparty/SortFilterProxyModel/SortFilterProxyModel.qbs" as SortFilterProxyModelSources

Product {
    // ...

    cpp.includePaths: [
        sortFilterProxyModelSources.prefix
    ]

    SortFilterProxyModelSources {
        id: sortFilterProxyModelSources
    }
}