Workrave should allow some events through (such as muting)

Forwarded from: https://launchpad.net/distros/ubuntu/+source/workrave/+bug/38910

When workrave makes me take a break, it blocks *all* input. This means that while on a break, it is impossible for me to e.g. mute the volume if my phone rings or if I have an offensive song on when my boss walks in.

Certain critical events should be allowed through workrave's block. This should probably be user-configurable, but it seems like a decent start would be to allow the following through:

Volume control (including mute)
Power off / Sleep / Hibernate
Fast-forward / Rewind (useful if you still want to control your music while on break from coding)

A nice way to implement this would be to allow certain key combinations to pass through to the window manager. I have keys mapped in Gnome to pause my media player, so if I could get those keys to work, it would be more than adequate. I find that I end up postponing a lot of breaks just to pause my media player and restart the break.

There is a similar bug report in the GNOME bugzilla for gnome-screensaver:

  http://bugzilla.gnome.org/show_bug.cgi?id=422390

For me, the most important part here is suspend/hibernate events. I'm running Ubuntu 8.10, and I cannot put the computer to sleep during a workbreak. I once was about to leave the office, so I attempted to put the computer to sleep, but workrave activated right as I was doing that. Sleep failed, but the screensaver activated. Because of bug 763, I was unable to even quit workrave and had to just wait until the work break was over for me to be able to suspend the machine.

Related: I'd like to be able to lock my workstation when the workrave screen is up. I suggest adding a tab in the preferences with a table to fill in, with some good defaults:

| Name | Command |
------------------------------------------------
| Lock | gnome-screensaver-command --lock |
| Mute | amixer sset Master toggle |
| Suspend | echo -n mem > /sys/power/state |
| Pause | audacious --pause |

And then we'd get those buttons on the take-a-break dialog. That's probably a lot easier than trying to filter keystrokes.