Avoid creating a new partition with a used mountpoint.

We get the mountpoints already used by other partitions, and
disable the Ok button in the "Create new partition" dialog if
the user selects/writes a mountpoint which is already used.

We are going to do the same in the Edit partition dialog
after testing.

src/modules/partition/gui/PartitionPage.h

@@ -62,6 +62,8 @@ private:
     void updateFromCurrentDevice();
     void updateBootLoaderIndex();
 
+    QStringList getCurrentUsedMountpoints();
+
     QMutex m_revertMutex;
     int    m_lastSelectedBootLoaderIndex;
 };