config/input: Invert movement for all pointer devices, not just touchpads

[m-matovic]

Describe your PR, what does it fix/add?

Use m_flipX/m_flipY to invert movement for all pointer devices, not just touchpads.
Move the flip_x/flip_y config options from input::touchpad to input and load them for all pointer devices.

Is there anything you want to mention? (unchecked code, possible bugs, found problems, breaking compatibility, etc.)

If the changes are merged, the wiki page Configuring/Variables should be update accordingly (I'll do it).

Is it ready for merging, or does it need work?

Ready

[vaxerski]

keep touchpad, don't break. Fall back to global if touchpad is unset.

[m-matovic]

I am a bit unclear on why touchpads had these properties unique to them? If they're just pointers with a size, why does having a size make them elligible for flipping but other pointers not?

Is it not more straightforward to leave the flip config options in global scope and load them for all pointers? They can be overriden in per-device configs.

I will return the isTouchpad property in any case, even if it's left unused right now.

Will make changes tonight.