PowerStatus: Add logic to automatically install in Deskbar.

Modeled after NetworkStatus. Also add it to
default_deskbar_items.sh.

Fixes #18748.

Enable KeymapSwitcher automatically for some locales.

Change-Id: Ie3b49be07dd02227850cb0d7d2a7befca7992668
Reviewed-on: https://review.haiku-os.org/c/haiku/+/2597
Reviewed-by: Sergei Reznikov <diver@gelios.net>

Moved per user post install actions to first-login script.

* The same mechanism (and the same PostInstallScript) is used for this.
* If a file first_login exists in ~/config/settings/boot, the first-login
scripts are launched, and the file removed.
* This fixes adding the deskbar tray icons even when there is no Deskbar
running yet (for example on first boot when the FirstBootPrompt
starts), or, IOW bug #12275.