Improve SIL About Box

- fix display of (default) SIL About box in test app (WinForms
  browser only works on Windows)
- some refactorings

Change-Id: I9279a9c82f60801d5679220f08c64128502a9290
Work around for issue #46

Fix SIL About Box to display HTML again. It was broken since the
introduction of the XWebBrowser.AllowNavigation property. This is
a workaround bug #46 (https://github.com/sillsdev/libpalaso/issues/46)
and should be reverted once that bug is fixed.

Change-Id: I842dfa7b2f23bbd6bd365ade9ce2c4147b518f95
Improve web browser to work with Geckofx 29 on Linux

- allow to use geckofx assemblies from GAC

Change-Id: Iaa3997ec9d52105677374fe4a83be6e9fc81427b