gtkui: improve performance of album art drawing in playlist

This patch improves the performance of album art drawing by doing the following:

- instead of calling the album art column code for every group item (ddblistview.c (old): 770) it is only called once (ddblistview.c (new): 824) (this also gets rid of some drawing artifacts when fetching the album art data)

- drawing code in plcommon.c 254/247 gets called less often when groups are pinned due to more accurate boundaries

- get rid of multiple calculations (grp_height, grp_row_y)