Add web caching options

Add web caching options to enable/disable, set cache size and force
cache usage without network request  (to be implemented in a future
commit.
Force use of network cache if enabled

If user has set the Force Cache option, then if it is a GET request we
check to see if it is cached, and set cache control options accordingly.

If we are setting QNetworkRequest.AlwaysCache, then we will not be
sending a request to a web site, so we will avoid resetting the last
request time.
Adjust default queuing priority for refresh

In order to prepare for giving UI refresh web requests priority over
batch web requests (like Add Folder), move most GET requests to the
low-priority queue. Important puts it at the front of the queue, so
should be false in most cases in order queue FIFO.
Give priority to UI Ctrl-R Refresh

Give priority to Ctrl-R refresh, and  ensure that MB requests are NOT
returned from the cache (like refresh in a browser).
Move web methods closer to where they are used

Just for code readability.
Address review comments
Secure personal data by using SSL for mblogin

When sending requests to MB with mblogin, use https port 443 instead of
80.

Note: Since headers should now be encrypted, there should not be a
problem with userid and password in the headers rather than using
QNetworkAccessManager authenticationRequired signal.
Change to use MB rather than MiB as per zas comment
Adjust coverart priorities to fit new approach.

Coverart webservice requests are now priority=False, important=False so
that CAA and Amazon requests are processed FIFO.
Re-add Pending Web decrement

Seems to have been deleted by accident in the rebase.
Fix potential rate limit error