Add easier ways to query/manipulate labels in the project

[PJEstrada]

Current issues with labels:

1. We currently cache labels on memory, and when adding new labels we are not adding them to that cache. So either we remove that cache and always query the backend for labels or we add the labels to the cache when creating a new one.
2. It would be nice to have a project.get_labels() function that can give us a list of the labes in the project.
3. When adding a new label, it would be nice to add a flag to check for duplicates as default. The user can explicitly deactivate it if he wants to create duplicate labels on the project. We already 99% of this done in the label_new function but this is not considering new labels created on the script. So maybe we'll need to add get_label_file_dict in here.
4. There is no way to delete labels using the SDK
5. The get_label_file_dict is just returing the first label with the name. But in the case of duplicates there is no way to know what are the 2 label ids associated with the same name.