Column grouping in search box

This adds column grouping in the search box, so the playlist behaves just like the main playlist.  This fixes issue #891 (album art in the  search window) and #1194 (crash when playing column is included in search dialog without a playlist widget visible).
Initialisation of some shared playlist resources (eg. pixmaps, theming, grouping) is moved to plcommon.c and occurs whether or not a playlist widget is configured.
Added a call to free the buffering pixmaps, a function which existed before but wasn't called.  Maybe there was a reason?
Added the group-by section of the header context menu in the search box.  Seems better than forcing users to go to the main playlist.
Possibly the main playlist and search playlist could behave and be configured separately, but for now they are identical.