Deprecated GTK Function Replacements

[Michael McConville]

Can someone with more GTK experience than me review this?

	https://hg.pidgin.im/soc/2015/mmcc/main/rev/3b91ffef050b

The preproc conditions are pretty ugly, but in GTK versions <3.16
gtk_misc_set_alignment() is deprecated and there's no alternative. The
conditions will make it easy to pull out the old code once our minimum
version gets bumped to 3.16 (it's currently 3.14).

I tested it with 3.14.13 (Ubuntu 15.04) and 3.16.4 (Ubuntu 15.10) and
didn't notice anything wrong.