Media upload bridge method refactor

[SergioEstevao]

At the moment we have the following bridge methods for media:

export function onMediaLibraryPressed( callback )
export function onUploadMediaPressed( callback )
export function onCapturePhotoPressed( callback )
export function onImageQueryReattach()

They are all related to media, the first three are used to trigger a media picker. So I'm proposing to rename them to

export function onMediaLibraryPressed( callback )
export function onMediaUploadPressed( callback )
export function onMediaCapturePressed( callback )

If you notice the signature they look very similar and only the source of media changes so we can go a step further and refactor them to a single method

export function onMediaPressed( source, callback )

where source could be one of the following values: SITE_MEDIA_LIBRARY, DEVICE_MEDIA_LIBRARY, DEVICE_CAMERA.

the last method is related to resync of media uploads when going back to an ongoing post so we could rename to:

export function mediaUploadResync()

What do you think? any problems with these names?

[mzorz]

Closing via #516 and WordPress/gutenberg#13554