Ability to assign a key for quick screenshot saving

[Korb]

By default, the screenshot is saved to the folder of the open z80/bin/scr/tap/sna file with a name corresponding to the current date and time.

Alternatively, the user may be prompted to specify a folder for automatic saving. But then the name of the created files must include the name of the app opened in ZX Speculator, so that later it is clear which applications correspond to which screenshots.