Display bluetooth connection and enabled status.

[riban-bw]

Current behaviour is to show a white icon when bluetooth is both enabled and connected otherwise to hide the icon. It would provide more information and maybe be more intutive if the icon was hidden when disabled, showed low-lighted (e.g. grey) when enabled but disconnected and showed highlighted (e.g. blue) when enabled and connected. The code overhead is minimal and the screen space is identical whilst providing some useful info. No need to go into the settings to find out if bluetooth is enabled. Instantly see the enable and connection status from most faces / apps.

I have submitted PR1974 that implements this but figured I should raise it as a feature request here.