Implement cross-platform browser wrapper

The XWebBrowser control can be used instead of a WinForms WebBrowser
control and underneath uses either a WebBrowser control or
GeckoFxWebBrowser control.

Change-Id: Idad22282b869b25bff3955f12c12b0ee942ef777
Add SilAboutBox and ShowReleaseNotesDialog to test app

Change-Id: Ib591f27415091cee09e3cce5a89cb2516e29aa82
Use cross-platform XWebBrowser instead of WinForms WebBrowser

Change-Id: I2e3f57c96fae5bcc528017b3d61e0adc31069ee1
Add GeckoFxWebBrowserAdapter

Change-Id: Ib129f45bc61338b0df1acc25c644c8dbc21b4cfc
Merge branch 'feature/geckobrowser' of https://github.com/ermshiperete/libpalaso into ermshiperete-feature/geckobrowser

Conflicts:
	Palaso VS2010.sln
Merge branch 'ermshiperete-feature/geckobrowser'