Add licence text to ui files.
Do not show tabs bar when there is only one bar at the editor panel and the main window notebooks.

It adds a signal when a tab is added and removed and it enables or
disables the show_tabs property.
Add text wrap on editor pannel.
Editor tabs are rebuilded every time a row is selected.
Change organization of source code for MessageFragment and TextTag classes.

MessageFragment was previously know as MessageMark and it
was a iterator specific class. It was moved to file and
messages related classes file and some new useful properties
were added. TextTag has been moved to a new file and it has a
new contructor "from_message_mark".