System message variable display time + setting option

[breadoven]

Closes #6725.

Provides setting for system message display time when multiple messages present. Also incrementally increases message display time for messages longer than 15 characters. Will double display time for messages 30 characters long. Variable time only works when armed.

[breadoven]

Tested in flight and works as expected. With a 25 character message as the longest message being displayed a setting time of 1000ms was increased to 1666ms.

[avsaase]

What is the display time of the shortest message?

[breadoven]

The display time for all messages, regardless of length, increases when one of the messages is > 15 characters. If no message is > 15 characters then display time is fixed at the setting display time (default 1000ms, same as current time). The setting has a minimum limit of 500ms. This change doesn't vary the display time per message as was suggested in the linked Issue simply because it's not easy to do given the way the message cycling is set up overall, So this is a compromise solution.

[Jetrell]

@breadoven @DzikuVx Any chance of this being merged? Given the ever increasing number of System Messages and their length, with little time to read and comprehend what just flashed before our eyes.

[breadoven]

Didn't think there was that much interest in this given it's a compromise and maybe not much of an issue for most people anyway. Still it does work so could be merged once updated to fix the conflicts.