Removing the Pidgin Theme Editor plugin

[Michael McConville]

Removing this plugin would be a good first step in simplifying our theme
code. Here's a diff in my repo:

	https://hg.pidgin.im/soc/2015/mmcc/main/rev/e83f409e347e

(It's just deletes the plugin's files and removes it from Makefiles and
translation files.)

Also, here's a valuable comment from its author (Sadrul) about the fact
that some of our theme abstractions are probably unnecessary:

	https://hg.pidgin.im/soc/2015/mmcc/main/rev/e83f409e347e#l3.173

Thoughts? Objections?