DiskUsage: Volumes in tabs, node-monitoring

[humdinger]

This is hrev29397.

With the drop-down menu to the top left you can choose the volume to examine. You can also switch between volumes while one is being scanned.

If tabs were used instead, we could do away with the whole bar at the top. Just one tab per mounted volume and a button "Scan volume" in the middle of every tab's view if it's not already scanned.

If volumes or folders are drag&dropped into DiskUsage, the tab of the according volume would pop to the front and the location would change to the dropped folder and the scanning starts.

With node-monitoring newly mounted volumes can be added automatically as tabs. Also node-monitoring would remove the need to rescan. It's the Haiku way. :)

[stpere]

should be done in hrev38958. Closing for now, yell if insufficient :)

[humdinger]

Beautifully done, Philippe! One thing that may be improved is the little too much vertical spacing in the status bar at the bottom. Another thing: increasing the window size does flicker quite horribly.