Move SMS App helper functions to their own file

Summary

I was lazy when setting up the sms app helper library and it was already starting to feel bad. Now, instead of exporting some random qml model which happened to have helper functions, this patch exports a small helper class as a library.

Test Plan

Run ./test/testsmshelper to verify test functionality.

Look at the resulting ./smsapp/libkdeconnectsmshelper.so (objdump -TC ./smsapp/libkdeconnectsmshelper.so | less) to verify that no "ConversationListModel::" methods have been exported, and instead that a small selection of "SmsHelper::" methods are available.