added Makefile
* src/filtering.[ch]
* src/folder.[ch]
* src/folderview.[ch]
* src/import.c
* src/inc.[ch]
* src/mbox.[ch]
* src/procmsg.c
* src/summaryview.[ch]
o added new flag need_update to FolderItem
o removed folder_table from various functions from tracking
folder updates
o set need_update flags in folder.c's move, copy and delete
functions and procmsg's flags functions
o added function folderview_update_items_when_required that
updates all folders with need_update set
o call folderview_update_items_when_required instead of
folderview_update_item_foreach
