Bug 1045209 - Remove media.gmp-gmpopenh264.path pref and set install path for OpenH264 plugin to <profile-dir>/<gmp-plugin-id>/<version>. r=unfocused,jesup

Certain usage scenarios, like enterprise roaming profile setups, lead to the installed OpenH264 plugin not being found anymore.
We install the plugin into the profile directory, so the fix here is to just store the install path relative the profile and not an absolute path.
We also now store the plugin in a version-specific subdirectory, which avoids further issues like e.g. Windows preventing updates due to locking loaded DLLs.
Bug 1045209 - Updating tests for new OpenH264 plugin path handling. r=unfocused,ted,jmaher