qolibri sometimes hangs for a minute or longer after clicking a hyperlink

[ivan]

This happens with both Qt WebKit and Qt WebEngine, so it appears to be unrelated to that change. strace shows qolibri populating ~/.cache/qolibri while it is hanging, but I haven't found the actual cause yet.

One set of repro steps:

1. load in all the usual EPWING files including 世界大百科事典
2. look up 「のと」
3. click the「 ⇒関連項目」hyperlink under 世界大百科事典

Tested qolibri 2.0.2 and 2.1.0 on Debian 9.5 stretch