Restrict suggested file filters to registered supported mime types

When using File > Open File or Project, and with Save As.
It was containing all mime types registered in Qt Creator, including the
default one from freedesktop.org, most of them being not interesting.

Task-number: QTCREATORBUG-17913
Change-Id: Ic5e49b27b566eb41419240b78a6cd99b233aeb07
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>

src/plugins/coreplugin/documentmanager.h

@@ -84,6 +84,8 @@ public:
                              const QString &fileName = QString(),
                              bool *isReadOnly = nullptr);
 
+    static QString allDocumentFactoryFiltersString(QString *allFilesFilter);
+
     static QStringList getOpenFileNames(const QString &filters,
                                         const QString &path = QString(),
                                         QString *selectedFilter = nullptr);