* Add a new socketutil module with a few helper functions that allow us to:
* Always use a sane User-Agent (previously, only Wikipedia did so)
* Set timeouts in an almost sane manner. Doing it explicitly prevents an interaction with KOSync that does crazy stuff I don't even want to try to understand.
* Unified said timeouts based on the request's intended usage (except for Wikipedia, which already had meaningful timeout values).
* Stopped using LuaSec directly, LuaSocket defers to LuaSec sanely on its own. Everything now transparently supports HTTPS without code duplication.
Option to synchronize files in local koreader folder with folder on Dropbox. All files that aren't on the local disk will be downloaded. Files with different sizes will also be downloaded and overwritten.
The download process can be paused or stopped at any time (similar to downloading images in Wikipedia).
Synchronize and settings are available after long press in dropbox account in Cloud storage.
Limitations: Only one folder (without subfolders) can be synchronize with one folder on local storage.
Option to show all files in cloudstorage also not supported by KOReader. We can also download any file.
To show all files in CS we need to enable option Show unsupported files in file manager (introducing in #5129)
Close: #5006
