Provide functionality to "Save/Load" a run

[claczny]

Currently, the results of the dimensionality reduction are stored in a temporary folder, the log is stored in the home-folder, and the sequence-input in some arbitrary folder. To improve reproducibility and ease-of-use, functionality to load/save a run should be provided.

For saving, the entire log of the current session, the 2D coordinates (points.txt), a copy of the sequences passing the threshold, and the associated (if available) annotation should be stored in a (via a dialog) user-specified folder. Loading should take the sequence file, the 2D coordinates and the annotation and display the resulting embedding in an according visualization without recomputing the embedding.

[claczny]

Would be good to have a notification once the saving is done. For big sessions, the user might accidentally close the app too early.