Update strings.xml

Added and translated into German the strings:
    <string name="action_add_dictionaries">Add</string>
    <string name="title_activity_file_select">Select Dictionary File</string>
    <string name="action_settings">Settings</string>
    <string name="action_goto_parent_dir">Up</string>
    <string name="action_reload_directory">Reload</string>
    <string name="msg_dictionary_already_open">Already open</string>
    <string name="msg_dictionary_added">Added dictionary from %1$s</string>