Software Keyboard on mobile

[ncannasse]

Following this issue ncannasse/picogpu#2

Currently we have TextInput.useSoftwareKeyboard / showSoftwareKeyboard / hideSoftwareKeyboard

I think instead we should have a software keyboard virtual API that doesn't rely on TextInput, but that could be used by TextInput, something like:

if( useSoftwareKeyboard  ) hxd.System.showSoftwareKeyboard(this.text, { onTextChange : (text) -> this.text = text, ... });

And we could implement this API on JS/mobile so all TextInputs are by default editable.