base-item: Style fixes
base-item: Serialize thumbnail creation for all items

We don't want to spawn 50 different threads to create the thumbnails
of all the items in the first batch. Instead, we want to create them
one at a time. Hence, use a class level lock instead of a per-object
one.
properties-dialog: Fix a compiler warning
build: Wrap glib-mkenums with $(AM_V_GEN)