Rework system of saving embeddings

[nvcleemp]

While fixing #28, I noticed that there are some improvements possible for the way in which we allow a user to save embeddings.

Old way
A users clicks one of the buttons (2D, 3D, folding net). Then a window pops up so the user can choose a format and a file name. After pressing OK the button is disabled, and when the user goes to another graph and he pushes the same button again, he gets no new window, but the graph is automatically append to the file he selected earlier.

Pro

• graphs can be collected in a single file

Contra

• a graph can not be saved in several formats
• some formats do not support multiple graphs in one file: these files can become corrupted

New way
A users clicks one of the buttons (2D, 3D, folding net). Then a window pops up so the user can choose a format and a file name. After pressing OK, the graph is output and the writer is closed.

Pro

• a graph can be saved multiple times in several formats and files

Contra

• no way to collect multiple graphs in one file (the user can write directly to a file, but that will write all graphs to the file, and not just the ones that the user selects)

How it should be

• There should be a way for a writer to signal whether the format supports appending.
• When the user presses one of the save buttons, there is an option to write to one of the open files or write to a new file.
• If a writer supports appending, then it is not closed, but saved in a list with open files. In the other case, it is just closed.