Inspect when InputSystem initializes the keyboard.

[psuong]

A user has reported than an InputSystem can fail to initialize the keyboard. This is hard to reproduce but it is a possible scenario. While NimGui can still run - unfortunately TextFields can no longer work as intended.

Immediate TODOs:

• Add in a state to see whether or not the keyboard has registered TextInput
• Test on 2020.3
  • Mac
  • Windows
• Test on 2021.2
  • Mac
  • Windows