main: Fix error handling when deleting items

First, the check for success when removing using grilo was reversed,
then the success variable could be used unassigned when no URI
was set for a particular GrlMedia.
main: Block selection changes during deletion

Don't act upon selection changes when we're removing the selection.
We'll get out of the selection mode straight after the deletion
finishes.
main: Don't use a stack-allocated GtkTreeIter

This usually won't work as expected.
main: Better debug when removing items
main: Get out of delete_foreach() if the path isn't valid

Just being safe.
main: Don't remove items by hand if source supports removal

There should be no need for us to remove the item by hand from the
tree store if the source supports removals, as we should be notified
of the item disappearing.