Add blueman apapters config to Xfce settings dialog

This partially addresses Issue #25

Making the dialogs pluggable into Xfce's settings manager is
probably not doable right now, given that Xfce is gtk2 and
blueman is gtk3.

Closes #152
Remove misleading plugin conflicts re NMIntegration

Closes #155
Clean up blueman/plugins/applet/AppIndicator.py
Remove leftover from custom status icon

AppIndicator seems to be broken since 50733a4