View documents containing question mark characters

Construct a QUrl with the static method 'fromUserInput' to tolerate
characters like question marks as part of (local) file's name.

This commit is a manual forward-port of commit 5ce346fe5518cd2835e
from branch 'master' which in its turn is a forward-port of commit
0269321a6e3016305ba7 from branch 'kbibtex/0.9'.

CCBUG: 401553