Just like it was until now, the default filtering should not be all
files, but only relevant files (i.e. XCF when saving, exportable images
when exporting and loadable images/XCF when opening).
Now all files will only be available through the "Show All Files"
checkbox.

This is simpler than previous implementations where the list was
proposing "All Files", "All Images" and "All XCF/export images". That is
just too much.

With this default, I get the "All Files" checkbox out of the expander so
that it is visible immediately even when the format list is unexpanded
(you don't want people to get pissed when not finding how to display all
their files).