added support for multiple files

xdot now can open multiple files. The open files are organized in a
linear list, which can be browsed with forward and backward
buttons. Files can be closed and are correctly inserted when selected by
the file browser.
menu for open files

Right click in the drawing area will open a context menu, with all open
files to select from.
keybindings for next and previous file

[ and ] are now keybindings for the next and the previous open file.