It's modestly faster, significantly more type-safe, and doesn't assert
when there's too few arguments, which causes problems for plural forms.
Closes#1733.
Most of the dialogs in Aegisub have a public interface consisting of a
single function, so there's really no need to expose the actual dialog
classes to the rest of the program. Add dialogs.h with the declarations
of all of those functions (most of which are just ShowFooDialog()), and
kill a million other headers.
