For certain reasons, it's always been hard for general players in Chinese mainland to establish stable internet connections with GitHub, which is the main cause for me to make this PR. Besides, I also think it would be useful if Catapult could install from a local build.

To the point, this PR:

1. Adds proxy support
   All HTTP requests now follow the proxy configured in Settings tab. Options include Disabled (default), Enabled and For Downloading Only.

2. Adds cache options

   1. If the file to be download is already there, don't download again. The added option Ignore download cache alters this behavior. This avoids unnecessary downloading. Also I can manually put zip files there now, when downloading isn't really an option.
   2. Also adds an option Keep download cache to not delete the zip file after extracting.
   3. Moves the download directory from program base directory to cache, to avoid clutter.

   Though it still lacks a part named "detect cache when fetching available builds" to make "offline mode" a real thing, but persisting cache can be the first step, I think. BTW the cache also does help in checking potential problems caused by corrupted files.

3. Refines Chinese locales
   Might not be the best, but certainly better I would say. I don't know how my English really sounds like but I'm confident in my Chinese. Fixes some mistranslations caused by polysemy; changes some terms and expressions - majorly the Game tab.

All changes have been tested locally, but it's always better to test more.