make HighlightUsagesExtension public
add ShowRuler to TextEditorData

In the same spirit as TabsToSpaces, allow extensions using
TextEditor to forcefully disable the ruler.
Add support for local HighlightCaretLine

If one uses DefaultSourceEditorOptions.Instance.HighlightCaretLine
to enable CaretHighlighting, it enables it for all the files in
the editor instead of just that local session (document opened).

This patch enables the editor to HighlightCaret for one specific
document which would be needed.

Signed-off-by: Aaron Bockover <abock@xamarin.com>
Document: make OnDocumentParsed protected virtual
Allow subclasses to modify loaded/saved text

Hypothetically, a subclass of SourceEditorView might want to store metadata
in a hidden header area of the file. Overriding ProcessLoadText gives the
subclass an opportunity to extract said metadata, and ensure that the text
loaded into the Document excludes it.

Similarly, ProcessSaveText would allow this subclass to inject updated
metadata into the file text just before the write occurs.
Let subclasses modify text in LoadNew, too

Missed this spot in the last commit.