Open recent files on the menu

This patch adds an "Open Recent" menu to keep track of most recently used files (up to 6). Almost everything that could be opened by menus, command-line and drag&drop operations, and later able to be opened with utils_open_file().

These links need persistence between sessions, so they are stored in the settings file (keeping care of autosaving flag).

On widget UIs, I haven't see a way to dynamically build submenus, so I use a selection dialog that does pretty much the same thing.

On GTK+ UI, the GtkUIManager does a nice job merging and unmerging portions of UIs. There is a GtkRecentManager object that manage the persistence and nicely integrates with desktop managers, but it would need mime-types and libgio so I have shelved for now.

Also has been implemented on win32 UI.

Opinions would be greatly appreciated.