Analog button and keyboard tester.

Added analog button indication for those buttons that may have use for it
- primarily L2/R2, but support was added for all buttons where at least
one controller is known to support it. Added also core option to hide
mismatching inputs where analog value is not sent (like keyboard
hotkeys assigned to retropad buttons). Analog button inputs are not
set up for remote transmission, only for local test.

Analog axes have now also dynamic coloring.

Added a keyboard tester screen which includes a standard 102-key PC
keyboard + extra blocks for all RETROK_ values present in the code.
Screen adapted from DOSBox-Pure onscreen keyboard with permission.
Keyboard button A+B switches between the two screens. Keyboard
inputs are not set up for remote transmission, only for local test.
Core option added to select start screen.

Other small improvements: core reset will take new option values,
input driver applies received values more carefully.