WebUI: Assign a version to the Web API and allow to get it

New GET methods:
 * version/api: current API version
 * version/api_min: min API version supported

The API version is an integer and it's returned as plain text.

Whenever the API is changed, the api version is increased.

Whenever the API is changed in a way that is not backward compatible,
api_min is increased.
WebUI: Allow to retrieve qBittorrent version

New GET method:
 * version/qbittorrent: qBittorrent version

Closes #1872.