Added fnc. get_file_in_data_dir

Returns the filepath of a file in data directorys.
Added fnc. so_show_message_dialog that shows a modal dialog.

Simpel wrapper for Gtk2::MessageDialog.
Added fnc. to open file by mimetype
Added fnc. to copy meta informations
Added a dialog to remove meta information
Added dialog to rename and move files
Added cbo. to show only tags by given group.
Wrote README
Added new screenshot