WIP: Rework AT sequence/command handling, and bundle SMS send/store commands in AT sequences

This MR ports the AT command/sequence logic to use GTask, and also wraps both things into a new "operation" context. The MMBaseModem now has a queue of operations, which are sequentially processed (e.g. one AT command operation will not be started if the previous AT command or sequence operation hasn't finished yet).

This is particularly useful to avoid getting AT commands embedded within the commands configured in a given sequence, and therefore fixes the issue with the SMS send/store actions which are now bundled in sequence operations.