Show "background" applications which currently have window(s) open

[waddlesplash]

Currently Deskbar hides applications which have the B_BACKGROUND_APP flag set from the Deskbar completely, i.e. even when they have windows showing. This should be changed so that they appear in Deskbar when some window is actually open.

In addition (perhaps this deserves a separate ticket), when a BAlert is the only thing an application has open, it should likely appear in Deskbar's window menu. (debug_server encounters both of these case at once, I believe.)

[jscipione]

From ​https://www.haiku-os.org/legacy-docs/bebook/BRoster.html#BRoster_GetRunningAppInfo

B_BACKGROUND_APP. The application won't appear in the Deskbar's application list.

So this would be a change to how BeOS was designed to work but maybe that is ok.

[waddlesplash]

Right now, non-BACKGROUND apps appear in Deskbar when they are running, even if they have no windows open at all. This change would make them appear in Deskbar only when they have windows open, but then hide them otherwise. So yes, a change from BeOS, but not a massive change.

[pulkomandy]

Some apps actually rely on this flag to hide from DeskBar while having a window (for example LnLauncher). Please don't change that, it would be annoying for such apps. If an app declares itself to be a background app, it probably has good reasons to do so.

I think the correct solution is elsewhere: applications in that case should make sure their alerts are B_FLOATING_ALL and so they can't be lost under other windows. Then you don't need DeskBar to locate them.