main/postmarketos-ui-xfce4: further improvements

Changes:

• Replace "Battery Monitor" by "Power Manager Plugin" (#1317 (closed))
• Fix the lower panel
• Change clock format from "1:23" to "01:23"
• Make font DPI changable for device package post-install scripts (#293)
• Reset list of keyboard shortcuts to default
• Remove brightness-switch properties from configs

More detailed descriptions on the changes can be found in xfce4-phone!8 (merged).

This MR is marked as a draft because of the workflow:

• First xfce4-phone!8 (merged) needs to be approved, merged, and someone has to create a new tag 0.5.0.
• Thereafter I'll change this MR to the new tag ($pkgver) and mark it as ready.