Put on-screen messages in a queue, display them one at a time

[ba58smith]

FIXED: It's crashing when the msg_btn_cb() function is called. It's supposed to display the next message in the pendingmessages std::list, but it crashes as soon as any element of gui is accessed. See the detailed notes on the most recent commit.

[ba58smith]

@JohnySeven, I think it has all the functionality that I wanted:

• Messages are queued and displayed one at a time, rather than just making a "stack" of them on the screen.
• Messages show the time they were generated, the number of times they've been generated since last cleared, and the number of other messages there are waiting to be displayed.
• Every time a message comes in, its timestamp and count are updated: in the queue, and also on the screen.
• The number of unread messages is updated on the screen each time a new one comes into the queue.
• A "long" vibration happens when the first message is displayed, and a shorter vibration happens as each new message is added to the queue.

The display of the messages might be made prettier, but I think that's for another PR. Please review this one for my coding - I know it works, but I'm not sure I've done everything as efficiently as I could.

After this is merged and we've both used it for a while, I will go back and remove all the logging messages.

[JohnySeven]

I've tested it and it really works very good! Tested it with my notifications sender script that I'm using and I like it! :-) Merging it.

[ba58smith]

You have a "notifications sender script"? Something that can simulate notifications coming to the watch from SK?

[JohnySeven]

I'm sorry I've missed your comment, yes I do have it, it's in C# and it basically sends UDP packet to server to update notifications.{anyname} data. If you want I can make simple Console app that you can run for testing.

[ba58smith]

No thanks - I don't need it anymore. And I really didn't need it all that much when I was testing. With a small butane cigar lighter I could make one of my temperature sensors get hot enough to throw an "alarm" whenever I needed one, and if I disabled my solar panels for just a few minutes, the battery voltage would drop low enough to throw a low voltage alarm. And if I needed 3 different alarms, I just turned off my wifi, which triggered that alarm.