GUI: Fix text alignment for the version in the launcher.

Formerly in LauncherDialog::reflowLayout an incorrect way to query the acutal
text alignment was used for the static text widget used for the ScummVM
version.
GUI: Center Launcher.Version in all themes.

Now that we actually use the textalign field of Launcher.Version the version
would be left aligned by default. This looks odd for the classic theme and
the low resolution version of the modern theme and is contrary to the old
"default" value, so I decided to center the string explicitly again.