Make plugin file search paths backwards-compatible

As part of Bug 1465107 (specifically, commit 5d7bdaea24b4d), we rearranged some of the search paths for theme files. We added "plugintype" to the search path for plugin-related files stored within a theme, and we removed "static" from the path for all the things that used to be stored under the "static" directory.

We did this in order to make the organization of core plugins more self-evident, but it had the unfortunate side effect of breaking all the third-party plugins that hadn't been reorganized to the new standard.

It's pretty easy to fix this. We already search a few different places for each file. All we have to do is add the old locations to the search path as well. Then the majority of pre-15.10 plugins will continue to work seamlessly in 15.10, making this major version change easier to handle.