Remote retropad display update

[zoltanvb]

Description

Refine the display for Remote RetroPad, as a preparation for using it as a test tool:

• added display for analog axes
• all inputs that were hit at least once, receive a light green color
• removed useless extra button from the middle (could never be sent)

Related Issues

Maybe #16263 - this PR makes no change in the functionality, but I could verify that all buttons and axes can be triggered with current implementation over network as well, though it did look unstable.