Apply syntax highlighting for source document types

[DaveJarvis]

Add a stylesheet that applies syntax highlighting to the editor whilst editing text-based documents. Currently there is a MarkdownEditorPane, which is instantiated by each FileEditorTab. This would have to be made generic with respect to a particular editor type. Examples: EditorPane<Markdown> and EditorPane<AsciiDoc>.

The key line for highlighting would resemble:

textArea.getStylesheets().add( STYLESHEET_MARKDOWN );

See MarkdownEditorPane::initEditor for details.

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There has been interest expressed in having syntax highlighting for Markdown. This may require some performance investigation because syntax highlighting may be CPU intensive. Would rather have real-time preview than a colourful editor. That said, it may be that some sprinkling of syntax highlighting would not bog down the CPU.