General code refactoring

[NicolaSmaniotto]

All these changes should not affect the functionality of the plugin.

Changes so far:

• [x] Changed how the closest thumbnail is found (useless optimization, but I don't see why not)
• [x] Simplified nil comparisons throughout
• [x] Removed some parentheses in function calls (though it may confuse someone not used to lua?)
• [x] Moved is_remote inside the thumbnailer
• [x] Make thumbnail generation depend on total thumbnails
• [x] Use mp.command_native