For phase 1 of the plugin manager, the ability to toggle off/on plugins
to be loaded at launch is provided.
This commit adds a new Plugin Manager dialog which can be accessed from
the Tools menu, which shows a list of all installed 3rd party plugins
with a checkbox to toggle them off or on. If a change is made, the user
is prompted to restart OBS. To allow this, the plugin manager uses a
json based config file stored in the OBS config directory. Additionally
for sources in the source tree, a sample UI has been provided that
indicates any sources whose parent module is disabled, by turning its
title red.
