logic: add name delimiter to NameHierarchy

* NameHierarchy now looks like this: <delimiter>\tm<all the rest>
* made new baseclasses for SettingsMigration and SettingsMigrator
* Use the migration system for SqliteBookmarkStorage as well

src/lib/settings/migration/SettingsMigrationDeleteKey.cpp

@@ -0,0 +1,15 @@
+#include "settings/migration/SettingsMigrationDeleteKey.h"
+
+SettingsMigrationDeleteKey::SettingsMigrationDeleteKey(const std::string& key)
+	: m_key(key)
+{
+}
+
+SettingsMigrationDeleteKey::~SettingsMigrationDeleteKey()
+{
+}
+
+void SettingsMigrationDeleteKey::apply(Settings* migratable) const
+{
+	removeValuesInSettings(migratable, m_key);
+}