Fix RH #633169 again.
https://bugzilla.redhat.com/show_bug.cgi?id=633169

This had been reported downstream at:
https://bugzilla.redhat.com/show_bug.cgi?id=1475369

An Exchange server, eventually also a GroupWise server, can break a GPG
encrypted message structure by changing it from

   multipart/encrypted; protocol="application/pgp-encrypted"
      application/pgp-encrypted
      application/octet-stream; name="encrypted.asc"

to something like:

   multipart/mixed
      text/plain
      application/pgp-encrypted; name="ATT00001"
      application/octet-stream; name="encrypted.asc"

where the text/plain part is empty and is added by the Exchange server.
The added code tries to detect such issues and if recognized, then
changes the structure to be able to decrypt the message as expected.

That was a memory leak, but apart of that it could also cause use-after-free
crash in gtk_widget_get_scale_factor() under _gtk_style_cascade_add_provider()
called from shell_window_constructed(), when creating a new shell window
for the second time, with the previous one already closed.

This part could cause crash on close of Evolution, due to use-after-free.

It makes the window to activate the Mail view, but it fails to switch
to the correct folder. The problem was that folder_name was actually
the folder_uri.

The later seems to be unreliable, returning no targets when pasting
into the composer, while the former returns the targets reliably.
Hard to tell what failed for the later, but as long as the other one
works, then prefer it over the other.