Add syntactic sugar for onBeforeInput & onBeforeInputCapture

[strobe]

this intended to address part of #689 which is not covered by #695

Add onBeforeInput & onBeforeInputCapture handlers .

As result following code should work:

input(onBeforeInputCapture := (event => { println(s"onBeforeInputCapture, data: ${event.data}") }),
      onBeforeInput := (event => { println(s"onBeforeInput, data: ${event.data}") }))

[strobe]

Later I should update this to adjust according on recent #695 changes

[strobe]

@shadaj sorry, I forgot to comment here on last week that this PR was updated

[shadaj]

Looks great!