QWebSocket: Add 'bytesToWrite'

Add the well-known 'bytesToWrite' function to QWebSocket. Previously
the only way to know how many bytes were left to be written was to
make a guesstimate on the overhead your message would get and then
keep track of the amount sent using the 'bytesWritten' signal.

The tests compare using '>' because there is overhead from the headers.

[ChangeLog][QWebSocket] Added the bytesToWrite function to
QWebSocket. Call this function to see how many bytes are left to write.

Change-Id: I82f17a98b582ee3bc02f0c47597b4a6717761f12
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@qt.io>
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>