Currently, plugin mimetypes are read from the browser process. This occasionally needs to happen for all plugins to create a database for determining which, if any, plugin is used for which mimetypes. Perhaps we could get lucky with RTLD_LOCAL dlopen if the plugin runs no functions and has no static constructors that use GTK/GDK, and GTK has no constructors, but this is delicate at best. A better approach is to read mimetypes from the plugin process and shut down these processes when done.

Launching a process is a pretty high price to pay, and currently we always do it synchronously at startup. We may need to do some combination of: * scanning for plugins and loading them all in a single process just to scan for MIME types * loading plugin info asynchronously instead of blocking everything the first time we need it