add mutex locks and unlocks to xviewer_list_store

[ygerlach]

This fixes #201

It seems XviewerListStore had a GMutex in its private section, but it was only used in xviewer_list_store_remove_thumbnail_job and xviewer_list_store_add_thumbnail_job i added it to most other XviewerListStore methods as i seemed fit. There might be some cases i overlooked.

I am not sure how well glib handles cases like this:

gtk_tree_model_get (GTK_TREE_MODEL (store), iter, [...], -1);

[stuff ... other threads may modify store]

gtk_list_store_set (GTK_LIST_STORE (store), iter, [...], -1);

would this cause glib to crash? or are such cases get handled gracefully? If not: then there might be some race conditions left.

(example from xviewer_list_store_thumbnail_unset)