Implement Screenshot Hotkey

[legoabram]

At the request of my brother, I implemented a basic screenshot mechanism into MelonDS. This just registers a new hotkey you can assign which will save the current frame buffer to a file at the native resolution of the renderer when the button press is registered. I've gotten this to functional for all graphic modes, and I've tested it on both Windows and MacOS with a couple of users from a nuzlocke forum who helped verify that it works correctly.

I'm happy to make adjustments as you need to better fit the architecture of the project, though I did my best to try and match it.

The last feature that was requested which I haven't yet added is a way to customize the actual filename. My basic thought process is that I'd add a simple templating structure, (Like %... in python, mmmm-DD-YY with strftime, etc.) and a text box that the user can edit to change this format string. Biggest issue is that I couldn't figure out where I would put such a dialog without adding a whole new window, so I want to ask you where this would make sense to go.

[Arisotura]

sorry for looking at this so late, but

looks interesting, minus inconsistent code style issues (opening brace should go on a new line...)

I'd merge this, however I'm not sure how well it will mesh with the OpenGL renderer rework I'm doing in the blackmagic3 branch, so I'll wait until that's finished and merged

[legoabram]

@Arisotura Worry not. I'm happy to rework/fix it once you've finished the renderer rework.

Though I still have my question regarding if/how to allow customizing the filenames.