[libcalamares] Add a clear() to GlobalStorage

While this isn't useful in production, it makes tests a lot
simpler to write since you don't have to wrestle with
creating new GS objects in order to start fresh.

src/libcalamares/GlobalStorage.cpp

@@ -88,6 +88,13 @@ GlobalStorage::remove( const QString& key )
     return nItems;
 }
 
+void
+GlobalStorage::clear()
+{
+    WriteLock l( this );
+    m.clear();
+}
+
 QVariant
 GlobalStorage::value( const QString& key ) const
 {

src/libcalamares/GlobalStorage.h

@@ -70,6 +70,9 @@ public:
      */
     int remove( const QString& key );
 
+    /// @brief Clears all keys in this GS object
+    void clear();
+
     /** @brief dump keys and values to the debug log
      *
      * All the keys and their values are written to the debug log.