Temporarily override status area with system messages

[bakkeby]

Here is a novel idea by @jzbor - allow important messages to temporarily replace the text in the status area for a few seconds.

This can allow for script messages or notifications to be displayed in the status area in place of what is normally there.

Example implementation ref. commit.

Technically the solution is similar to that of the fsignal patch in that it hooks into the X root name handling and uses different logic if a specific prefix is matched.

The time of the last important message is recorded and any other X root name updates are ignored if the last important message came through less than a configurable number of seconds ago. A separate prefix allows the important message to be cleared before that expiry time.

The restoring of the status text in this context naturally assumes that the status is updated frequently (e.g. every second or every few seconds).