Display many UI messages at a time

[alec]

In UI we've got messages of type 'loading' that are displayed until some action finished, e.g. on AJAX response. We've got also other messages (error, warning, notice, confirmation) that are disapear after some (short) time. Attached patch changes behaviour of displaying messages, so we can see many messages at a time. Messages of type 'loading' are grouped by label, so e.g. we have one 'Loading...' message even if many actions are run in parallel.
Patch doesn't handle all situations e.g. some forms submitting, where loading message is displayed continously, this changeset will also not backward compatible with some plugins that are using set_busy().

[alec]

A big difference in working with code after patch is applied is that now display_message() (and set_busy when called with 2nd argument) returns message identified which must be used to remove message (with hide_message() or set_busy()).
Didn't you see a difference? Go to mail task, execute messages search, a green box appear when searching is done, then quickly change search filter and execute search again (before the green box disapear). You should see two green messages boxes.

[alec]

Improved and applied in [ad334a12].