t-rec allow for keystroke capturing (toggleable during recording)

[sassman]

As a user
I want to show the keys I press on demand
so that people understand keys to use in a demo

• A keystroke (tbd) that does not interferes with the terminal nor the program should turn on keystroke capturing
• The same keystroke should turn it off
• Keystroke should be visually shown in the recording (gif/mp4) as a overlay
• the keystroke overlay should have the look and feel of a mac keyboard key
• the overlay should have some opacity to not hide everything
• the overlay should be configurable in size, default 60 x 60 px
• technically it would be good that keystrokes are bound to a time code, and live for a configurable amount of time, default 300ms
• a new keystroke should lead to a replacement of the old one, instantly
• also the keystroke image should be created separately, and not be blend into the captures screen image directly, this should be a matter of post processing, as is the decor and wallpaper application