opt: GlobalNetworkAccessManager

[shenlebantongying]

Things happens in the main event loop can share the same QNetworkAccessManager

#974

Other than AnkiConnector and iframehandler, all QNetworkAccessManager are created by MainWindow, then passed down to various places. This means, if the old code is correct, then obtaining the pointer from global (pretty much the same as the MainWindow) is also correct.

We can further remove the netMgr parameter from loadDictioanries/editDictionaries. And instead, let dictionaries obtain a pointer to the globalNetworkAccessManager during initialization.

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Random related things:

QThread::isMainThread in Qt6.8 qt/qtbase@a3d5011#diff-4bb5aaa078a4700cf1c8a805176a3d445f6b0408e526612cb0a13fb19f5bd1dbR429-R444