Force a repaint of the splash screen.

Ensures the image is painted promptly and does not wait for the event loop
to start. If the ApplicationWindow constructor takes too long then
the event loop paint events don't start and the image only gets painted
at the end.
Refs #10741
Remove the forced 1 second sleep statement from main.

It only holds up startup even and doesn't seem to be required any
longer.
Refs #10741
Merge pull request #100 from mantidproject/bugfix/10741_gray_splash_linux

Fix gray splash screen on Linux