Fix parsing the `proxy` and `proxy-host` command line option

**Fix including http:// in the proxy URL**
The proxy host parsed incorrectly, when it was typed with a scheme (http or https).
Now proxy can be specified with the scheme.

**Fix invalid type conversion**
`m_proxyPort` converts to its ASCII representation.
Use `QString::number` to include a proxy port properly.

**Issues:**
https://github.com/ariya/phantomjs/issues/11117
https://github.com/ariya/phantomjs/issues/10811