Rework code by using libmm-glib

Epic: https://source.puri.sm/Librem5/use-cases/issues/40, https://source.puri.sm/Librem5/use-cases/issues/44

Rewrite the plugin code by replacing g_dbus_proxy_calls with libmm-glib to make the code more rugged and handier to use for the implementation of further functionality, like delivery reports and the handling of offline SMS, etc.

Get MM to talk to the libpurple plugin via libmm-library

Edited May 16, 2019 by Andrea Schaefer

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Rework code by using libmm-glib