Add SortFilterProxyModel.qbs for inclusion into Qbs projects (#89)

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
        }
    }

SortFilterProxyModel.qbs

@@ -0,0 +1,63 @@
+import qbs
+
+Group {
+    name: "SortFilterProxyModel"
+    prefix: path + "/"
+    files: [
+        "filters/alloffilter.cpp",
+        "filters/alloffilter.h",
+        "filters/anyoffilter.cpp",
+        "filters/anyoffilter.h",
+        "filters/expressionfilter.cpp",
+        "filters/expressionfilter.h",
+        "filters/filter.cpp",
+        "filters/filter.h",
+        "filters/filtercontainer.cpp",
+        "filters/filtercontainer.h",
+        "filters/filtercontainerfilter.cpp",
+        "filters/filtercontainerfilter.h",
+        "filters/filtersqmltypes.cpp",
+        "filters/indexfilter.cpp",
+        "filters/indexfilter.h",
+        "filters/rangefilter.cpp",
+        "filters/rangefilter.h",
+        "filters/regexpfilter.cpp",
+        "filters/regexpfilter.h",
+        "filters/rolefilter.cpp",
+        "filters/rolefilter.h",
+        "filters/valuefilter.cpp",
+        "filters/valuefilter.h",
+        "proxyroles/expressionrole.cpp",
+        "proxyroles/expressionrole.h",
+        "proxyroles/filterrole.cpp",
+        "proxyroles/filterrole.h",
+        "proxyroles/joinrole.cpp",
+        "proxyroles/joinrole.h",
+        "proxyroles/proxyrole.cpp",
+        "proxyroles/proxyrole.h",
+        "proxyroles/proxyrolecontainer.cpp",
+        "proxyroles/proxyrolecontainer.h",
+        "proxyroles/proxyrolesqmltypes.cpp",
+        "proxyroles/regexprole.cpp",
+        "proxyroles/regexprole.h",
+        "proxyroles/singlerole.cpp",
+        "proxyroles/singlerole.h",
+        "proxyroles/switchrole.cpp",
+        "proxyroles/switchrole.h",
+        "sorters/expressionsorter.cpp",
+        "sorters/expressionsorter.h",
+        "sorters/filtersorter.cpp",
+        "sorters/filtersorter.h",
+        "sorters/rolesorter.cpp",
+        "sorters/rolesorter.h",
+        "sorters/sorter.cpp",
+        "sorters/sorter.h",
+        "sorters/sortercontainer.cpp",
+        "sorters/sortercontainer.h",
+        "sorters/sortersqmltypes.cpp",
+        "sorters/stringsorter.cpp",
+        "sorters/stringsorter.h",
+        "qqmlsortfilterproxymodel.cpp",
+        "qqmlsortfilterproxymodel.h"
+    ]
+}