Trivial typo fixes
Make some string data const

https://bugzilla.gnome.org/show_bug.cgi?id=144042
Remove an outdated comment

We're now marking the tiff loader as threadsafe, so
remove the comment that says it isn't.
Consistenly annotate format names for translation

The right macro to use in the fill function is N_(),
but some loaders were using _(). Correct this.