On Android, MotionEvent with 0 values might come for axes when connecting
a gamepad, even though a user didn't touch any axes. This is problematic
especially for tirgger axes, where the default value should be -1.
This change fixes the issue by adding a new state `axesReady` to check
if an axis is really touched or not. If an axis is not touched yet,
a button value for a standard (trigger) button always returns 0.
This change also removes an old hack to initialize axis values for
triggers.
Closes#2598
Refactors native standard layout so standard axes and buttons can be implemented using any of axes, buttons or hats.
This is required to be able to implement a native standard layout mapping for Linux, as e.g. shoulder buttons can be
backed either by an analog or digital input.
Precedes #2587
Updates #2052
