custom props: Fix possible segfault

Because there can be refreshers for custom properties running at the same
time in different threads, accessing property->{items,source} needs to be
under a lock.

We use LOCK_PROVIDERS_WRITE from app since those belong to providers.
Fix/silence some deprecation warnings
Silence warnings
provider-mru: Fix using uninitialized variable
provider-filter: Fix calling g_object_notify() w/out a property

In case we did emit a node-new-child there's no notify to emit.
Makefile.am: Fix missing filter-private.h
command: fix possible use of uninitialized memory

When adding commands we forgot to set data & destroy to NULL, but even
though they're not used for such commands, they get used when finalized.

If the memory wasn't set to NULL by chance, we'd segfault.
treeview: Fix using unexisting variable if GTK not patched
Bump to 0.3.0; Update HISTORY & README