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This is a rudimentary hack to avoid navigating away from the current page
while also allowing loading the external resources we need.

Related to #760

We don't need this anymore now that we rely on 0.47, which simplifies the
process of copying files to the build dir.

We need to clear this state or PyGObject will leak our preview
addin when the view is destroyed.

Our templates have moved away from this, and we can remove it from our
sources too.

Instead of controlling the transfer manager at the IdeContext
level, this uses IdeApplication as the owning instance. This is
useful because it allows us to use it from preferences without
having a context accessible.

As the project has grown, we've let things get a bit out of their
logical place. This does a bit of cleanup and tries to bring some
of the embedded resources closer to where they are used.

But more importantly, this allows us to clean some things up to
the point that we can move forward statically linking a bunch of
the plugins into libide. The plan here is to speed up the initial
loading by avoiding lots of disk I/O stats which are currently
hurting us.