[applet] Use section separator consistent with plasma-nm applet

Nate Graham requested to merge ngraham/bluedevil:consistent-separator into master

This commit makes the Bluetooth applet use a single line separator between the Connected and Available sections--just like the plasma-nm applet does--which is consistent with the master System Tray applet mockups (https://phabricator.kde.org/file/data/wiv5bs4ucvqnfiibxtjy/PHID-FILE-nnurs2z323lvwrjsavun/system_tray_w%E2%81%84headers_2x.png)

I'm aware that this technical implementation is a bit unpleasant, so I'm open to suggestions. FWIW, the way the first-section-delegate-hiding code is implemented in the plasma-nm applet is even uglier. :)


cc @teams/vdg

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