Big commit. (See details) This commit makes this project broken!!

 * Changed repository structure. Moved ui files and language files to
 data directory. Created gresources file in this direcory and modified
 paths from templates classes.
 * Substitute reveal plus ad-hoc widget by search bar and search entry widget.
 * Remove headerbar and notebook classes and implemented directly on window template.
 * Added visited variable to MessageIterators.
Remove files missed in the last commit.
Added files needed for autotools.
Delete lines of erased ui files and modified path for messagelist widgets.
Add first implementation of gettext-po bindings. It does not work.
Fix search with search bar.
Add and implement undo, redo, search next and serch previous actions.
Add modelines to every file and remove useless comments.
Fix undo redo action.
Added undoable action when set translation message.