* use libtool convenience libraries as much as possible
   (for all static libraries except Scintilla)
 * improves separation of language and user interface implementations
   (e.g. the Gtk widgets are not interesting for the rest of SciTECO)
 * the Curses popup widget can now be factored out of interface-curses.cpp
 * some common CPPFLAGS are now defined by ./configure via AM_CPPFLAGS,
   so they don't have to be repeated in each submodule.
 * fixed building the Curses UI: GTK_FLOW_BOX_FALLBACK conditional
   must always be defined.

 * depend on Gtk+ 3.10. If necessary older versions should also
   be supportable. GtkOverlay was already introduced in v3.2
 * A fallback for GtkFlowBox is compiled in if the Gtk installation
   is too old. This applies even to Ubuntu 14.04 which still runs
   Gtk v3.10.
 * the threading the Gtk UI is left as it is for the time being
   even though the synchronization mechanism has been deprecated.
   Alternative approaches have to be tried out and benchmarked.
 * Completely revamped the GtkInfoPopup widget.
   It is now as powerful as the Curses UI's popup widget.
   * A GtkOverlay is used instead of the top-level window hack
     in the Gtk2 version.
   * GtkFlowBox is used to lay out the columns of the popup.
   * I had to work around restrictions of GtkScrolledWindow
     by writing my own poor-mans scrolled window which handles
     size requests correctly.
   * The popup window no longer overflows the screen size,
     instead we scroll.
   * Scrolling pagewise is finally supported. Wraps at the
     end of a list just like the Curses UI.
   * Instead of using only two stock icons, we now use
     GIO to get file and directory icons for the current theme.
     This looks even better.
   * The GtkFlowBox allows selections which can be used for mouse
     interaction later. But this is not yet implemented.
 * Theming of the popup widget and command line is still
   not performed correctly.