api: Add TitlePart

[kezz]

This PR is an alternative to #320. With this PR, you would send individual title parts like:

audience.showTitlePart(TitlePart.TITLE, text("hello!"));
audience.showTitlePart(TitlePart.TIMES, times(0, 100, 15));

[kezz]

Just spotted some incorrect @since tags so have updated those and rebased to latest. For reference, the client doesn't care about the ordering of the packets, so the replacement implementation for the showTitle method is also fine.

[zml2008]

Could you add some API to this to map from a Title to and from its component TitleParts?

something like Title.part(TitlePart) and Title.title(PartValuePair...) or something along those lines?

[kezz]

Does anyone have any thoughts about sendTitlePart vs showTitlePart?

[kashike]

Probably sendTitlePart