Rename global functions

[onmyway133]

As a follow up to #89

• Add show/hide functions, which make it easy to remember. If we ever have more kinds of message, we can do the same way
• Make use of first parameter name in function to specify the type of UI, so we have something like

show(whisper: message, to: navigationController, action: .Present)
hide(whisper: navigationController, after: 3)
show(whistle: murmur, action: .Present)

[mention-bot]

@onmyway133, thanks for your PR! By analyzing the annotation information on this pull request, we identified @RamonGilabert and @vadymmarkov to be potential reviewers