Add Text Widget virtual class

[knowknowledge]

Epiar has a few text based widgets that all share similar interfaces, but are not currently grouped explicitly or superclassed by a virtual class that could formally define the interface.

Here are the widget classes that implement some form of text storage:

• Label: A single straight lines of text.
• Paragraph: A grouping lines of words in blocks. Currently aimed at presenting prose as the lines wrap at word boundaries.
• Button: A clickable word or phrase.
• Dropdown: A selection from a set of words or phrases.
• Textbox: A single straight line of text that can be edited.

See the UI_Lua::GetText? and UI_Lua::SetText? for an example of why we need a unifying formal interface.