Honor GTK theming in LO flatpak

...according to alexlarson, for now (until "eventually we'll have a sane dconf
in the sandbox") apps need to specify --filesystem=xdg-run/dconf,
--filesystem=~/.config/dconf:ro, --talk-name=ca.desrt.dconf,
--env=DCONF_USER_CONFIG_DIR=.config/dconf (where the first two are irrelevant
for LO due to its --filesystem=host).

This fixes <https://github.com/flathub/org.libreoffice.LibreOffice/issues/4>
"Libreoffice flatpak is not respecting user theme".

Change-Id: If5ed33e1b524bfa877b89f052c6067398b5600a3
(cherry picked from commit 12ba14c6)
Reviewed-on: https://gerrit.libreoffice.org/45806Reviewed-by: Stephan Bergmann <sbergman@redhat.com>
Tested-by: Stephan Bergmann <sbergman@redhat.com>

solenv/flatpak-manifest.in

@@ -392,8 +392,10 @@
         "--socket=wayland",
         "--socket=pulseaudio",
         "--filesystem=host",
+        "--env=DCONF_USER_CONFIG_DIR=.config/dconf",
         "--env=LIBO_FLATPAK=1",
         "--own-name=org.libreoffice.LibreOfficeIpc0",
+        "--talk-name=ca.desrt.dconf",
         "--talk-name=org.gtk.vfs.*"
     ]
 }