Improve getGuid method

[JonnyWong16]

Description

Improve the LibrarySection.getGuid() method so it doesn't need to iterate through the entire library to find a specific external guid.

This change "exploits" the "Fix Match" function in Plex which allows searching by external guids. We fetch a dummy object (the first media object in the library) and call the match function using the external guid. Then a library search is performed by using the Plex guid (plex://) from the match. Searching for a Plex guid is much faster since it is offloaded to the Plex server which supports filtering by the main guid.

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added or updated the docstring for new or existing methods
• I have added tests when applicable