Solve MultiLine TextEntry problem by adding a new TextArea widget

This is an approach to solve problems with TextEntry.MultiLine in a more clean way. Some Backends (Gtk for now, maybe other in the future) have different widgets for a single- and a multi-line text input (Gtk.Entry vs. Gtk.TextView).
There are some different approaches and ready solutions, but none of them is really clean (see mono/xwt#367 and discussions in mono/xwt#288, lytico/xwt#25 and some posts in Forum).

This PR solves the issue by deprecating Xwt.TextEntry.MultiLine and adding a new widget Xwt.TextArea (like in HTML). The most code has been moved to an abstract class Xwt.TextBox which handles everything except the MultiLine property. TextEntry and TextArea are subclasses of Xwt.TextBox and implement only their specific properties (new TextArea.Wrap and deprecated TextEntry.MultiLine)

It is important, that TextEntry.MultiLine has not been removed (only marked with the Obsolete attribute). ITextEntryEventSink is not used by any Backend anymore, but should still be functional (for third party implementations). TextEntry still maps the deprecated TextEntryEvent (replaced by TextBoxEvent), too.


PS: This PR contains mono/xwt#368! I accidentally checked out the wrong branch and realized that after pushing it to github. So this PR depends on mono/xwt#368 and I had no time to split them. SORRY!