Export VIBes figures to common graphics formats (PNG, JPEG, BMP, SVG).

Closes #10
*export* action in VIBes JSON: 

    {"action":"export", "figure":"my_fig", "file":"filename.png"}

Supports PNG, JPEG and BMP as raster format, and SVG for vector graphics.
Also added an export button in the VIBes main window, to export the figure currently selected in the tree view.