Merge pull request #80 from iv597/master

Allow Budgie Popovers to be passive (sets up stuff for Notifications support)
Add prefix=/usr instructions
Introduce legacy X11 tray.

This is reliant on NaTray, nabbed from GNOME Panel.I have no intention in
maintaining this longer than I have to, and firmly believe xembed should
be taken outside, tied to a tree and have both legs shot with nail guns
while slowly being tortured to the sound of failed Disney artists.

The main rationale in including this right now is the lack of agreed
"indicator" spec between major desktops, meaning we will support our
own applets primarily, xembed cruft as a last option and in the future,
appindicator.

People need to be able to access programs that still use the tray,
such as "nm-applet", xchat, etc. Now I've explained why this is here,
please, enjoy the bugs.
imports/natray: Caught out by copy/paste again.
gvc: Add workaround for broken Vala support (virtual method)
applets/status: Introduce volume control and unified popover

The unified popover (or "Aggregate menu") enables the quick changing of, and
access to, common desktop tasks. Right now this involves a link to the
settings, interactive control of volume, as well as an option to end the
session. This will be expanded in the future to support networking and
devices.
applets/status: Work around mark-business and show normal volume limit

This gives a visual indication of the maximum volume pre-amplification, so
users are instantly aware they are above 100%
applets/status: Use percentage reading for tooltip not dB
applets/status: Enable changing volume with scroll wheel

This commit enables users to change their volume very rapidly by scrolling
with the mouse wheel over the sound icon.