fix(windows): fix size of splash

Fixes #5195.
Fixes KEYMAN-WINDOWS-M0.

The splash screen for Keyman would crash on start with the latest
Chromium update. This fix resolves the issue, which related to the
resize-to-content code, by removing the resize-to-content code and
instead hardcoding the window size in the .dfm.

This is a significant simplification of the browser host and I think it
is worthwhile. The only place the resizing code was still used was in
the splash screen.