Fix truncation of "Bin?" and "Src?" column headers

Fix truncation of "Bin?" and "Src?" column headers

It seems that the the "Bin?" and "Src? columns in the PickView are usually not
quite wide enough, and display truncated with an ellipsis, e.g. 'B...' and
'S...', which is completely unintelligible.

Try to more correctly size these colums.  3*GetSystemMetrics(SM_CXEDGE) seems to
be documented as the default value for the margin of a header control, so text
length + twice that is the minimum header width needed.

ChangeLog:

2015-03-02  Jon TURNEY  <jon.turney@dronecode.org.uk>

	* PickView.cc (init_headers): More correctly size "Bin?" and
	"Src?" columns.