Improve SMS Plugin Functionality

Improve plugin startup (plugin/modem initialization)
Improve modem state handler (will potentially fix chatty#138 (closed))
Add an API function to provide MODEM status (will resolve chatty#94 (closed))
Add support for multipart SMS (will close #10 (closed))
Evaluate delivery reports and add API function (will close chatty#65 (closed))
Improve message-acknowledge handling
Fix smiley issue - libmm->mm_sms_get_text is limited to UTF-8 (will resolve chatty#136 (closed) and chatty#139 (closed))

Edited Jul 24, 2019 by Andrea Schaefer

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