Text Updating

Right now the EmojiLabel class gets initialized with the desired text and does not contain any mechanism for updating it.

The rendering logic can be broken out to an internal function called by init, and then be called from a `text` property setter as well.

An offshoot of this might be some kind of internal Bitmap cache that can store the recently used images so they can be re-used without having to read the file and load it again. Maybe the user could enable / disable it and use it when it makes sense for a given project based on repeated characters or emoji appearing in the text they are using vs. always being different.

Start at the EmojiLabel class initialization and its current rendering logic. Extract the rendering path so initialization and the text property setter can invoke it, then verify that changing text updates the displayed output; bitmap caching is an optional follow-up rather than required scope.

Written by the indexing model from the issue text.