Potential data corruption/deletion bug while creating new difficulty

[Brollyy]

46 Issue was caused by hanging autosave timer thread and map file references not being properly disposed of when closing the window.

Summary of the changes:

• Disposing of all the object references in MainWindow to avoid memory leaks and hanging threads when closing it.
• Implemented some guard rails for spectrogram generation, since it was being started multiple times when map was first being opened.
• Fixed an unusually large memory leaks caused by spectrogram generation trying to process too large BMPs.

Now the memory is manageable even for very large maps and is being (more or less) correctly released when closing a map and opening a different one.