Fix segfault during bookmark DND operation

Signed-off-by: Niklas Koep <niklas.koep@gmail.com>
nemo-bookmark-list.c: rename `clear()` function

...to `clear_bookmarks()`

Signed-off-by: Niklas Koep <niklas.koep@gmail.com>
Don't register bookmarks_breakpoint as gproperty

The bookmarks_breakpoint was previously registered as gproperty, but not
consistently treated as such (e.g., by directly assigning to it rather
than calling `g_object_set()`). It's enough to listen for modifications
of the respective gsettings key; we don't need to register the property
with the NemoPlacesSidebar class as well.

Signed-off-by: Niklas Koep <niklas.koep@gmail.com>
Restrict value range of bookmark_breakpoint

Make sure the value of the bookmark breakpoint is restricted to the
range 0..bookmark_count. This is necessary to make sure all bookmarks
are always shown, even when manually assigning a bogus value to the
sidebar-bookmark-breakpoint gsettings key.

Signed-off-by: Niklas Koep <niklas.koep@gmail.com>
Merge pull request #740 from nkoep/bookmarks-fixes

Fix a few issues with the new bookmarks handling code