move screenshot and blending toggle from keydown to keyup

Both F1 and F3 are handled on KEYUP.
Toggling blending and screenshots are handled to KEYDOWN: this makes them susceptible to "the holding thing", in which holding a key makes the KEYDOWN event fire repeatedly.

This fixes that "holding thing" moving those keybindings to KEYUP, which also makes the code tidier :dancer: 

Supersedes #139