Fix BUG causing `ctx thread` to ignore -p arg; Fix small memory leaks

ctx_thread.c:
* BUG FIX: load paths for the colour being used annotated
* ctx thread was effectively ignoring -p argument

path_format.c:
* added path_load_colour() function similar to graph_reader.c:graph_load_colour()

merged FileFilter dealloc()/close() methods
* file_filter_dealloc() -> file_filter_close()
* graph_file_dealloc() -> graph_file_close()
* path_file_dealloc() -> path_file_close()
* fixes several small memory leaks in ctx commands

graph_crawler.c
* unicol_path_cmp() now compares empty paths correctly