Replace Printer's boolean flags with threading.Event

[CRImier]

ZPUI/ui/printer.py

Line 22 in 6a74d74

Printer.skip_screen_flag = False #A flag which is set for skipping screens and is polled while printer is displaying things

This solution is old. Now I see that:

• Any simultaneously running Printers will probably interfere
• It's not thread-safe - so, we might miss the changes when the CPU load is too high (as it used to happen with Refreshers and other UI elements).

We can use threading.Event instead, just like we do in BaseUIElements.