ENH, WIP: Spectrogram mode, minimal version

[alexrockhill]

Fixes https://github.com/mne-tools/mne-qt-browser/issues/248.

Hey look, I have a minimally working version and didn't take that long once I got around to this.

Screencast from 07-10-2024 04:52:50 PM.webm

This isn't the all-in-one data analysis that @nmarkowitz was suggesting, it's just a different mode to view the data in for quality check purposes and if no one reads the help menu and knows to press "f", the browser won't be effected. How it works is that the TFR data is computed on the fly only for the times in view. Will add tests and clean up etc. but it would be nice to get feedback if anyone thinks of things. For instance, there is no way to tell the frequencies at the moment which seems like a bit of an issue. Also, might run slow on a less powerful computer but you can also limit your selection to a smaller time and channel range.