Make WatchFace timer dynamic

Hello,

I made a small change to WatchFace.cs in order to make the internal timer's period dynamic. 

The idea is to check at each tick if the UpdateSpeed of the face has changed and to set the timer's period accordingly. I had a use for this by implementing a StopWatch face. When the stopwatch is running, I need a very short update frequency. But when the stopwatch is not running, I can go with a longer frequency. I suppose this can have a positive impact on battery life.

By the way, I'm quite new to Git, so I think I made something wrong as the pull request shows 95 files changed :-(
In fact I only updated WatchFace.cs (commit Id cde534c and 5c91dc7).

Please let me know what you think about my proposal. I'm more than willing to get feeadback.

Philippe HERTZOG