Allow passing delay = 0 to elgg.system_messages() to disable fadeout

[brettp]

Will be useful for warnings (like the token refresh failed) that should be acknowledged.

[juho.jaakkola]

Before finding this ticket I had the idea of giving the messages an importance level and changing the look and behavior of the message depending on the level.

Example with three levels:

1. A simple looking message that disappears quite quickly on its own
   • "Blog saved"
2. The message is more prominent and stays visible longer
   • "Your blog post is missing a title and you should do something about it."
3. User has to close the message manually by clicking a cross in the corner
   • "Something went horribly wrong and here's a lot of important information about it: ..."

But I'm not sure which is better, a delay or an importance level.

Oh and here are two related tickets. These three should probably be combined into one.

• #4371 Modifying the system message delay time
• #1390 Option to disable auto-close for system_message and register_error.