Unifies the 'welcome' prompt with the about text variable.
Removed the TODO about SOME-KIND-OF-IMAGE-P.

Adding a dependency to fix something that is already /not/ broken
is just too much hassle.

It's also true that SDL-IMAGE:IMAGE-TYPE-OF exists, but fails
on TGA files.

As a compromise, I moved the ugly TGA test in LOAD-IMAGE-AS-CEL
into its own function: LOAD-GODDAMN-IMAGE.