collect all status section inserters in one place

All of the sections created by these inserters are used in the "status"
buffer.  They can also be used in other Magit buffers but that doesn't
change the fact they are "status" sections.

Some of these inserters were defined as part of the "Status Mode"
toplevel section, while others where defined next to the lowlevel
functions they make use of.  There are two ways to make that
consistent... this commit implements on of them.

....