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
Fix default property values in WebBrowser

Initialize default property values to match the values we get
from the Winforms WebBrowser. Without explictly initializing the
values we get differing values between WinForms and Geckofx
browsers.

Also fix build: when building with MonoDevelop the build occasionally
failed that it couldn't find L10NSharp.

Change-Id: Idf2d1dfd06c53f54e3ec08152bffd91571033db5
Fix SILAboutBox and ShowReleaseNotes dialogs

Set AllowNavigation to false and use Url property to load the content.
Note that even with AllowNavigation it is still possible to click
on links, even on Winforms WebBrowser.

Change-Id: I73b1afb282ad7cb730ab262b0458a5c3f3b2ac74
Make release notes button visible in test app

Change-Id: Iaec6a9ac2ae1c1fe3b6c27d24b6023d0c30a2f05
Merge pull request #47 from ermshiperete/feature/ImproveAboutBox

Improve SIL About Box