Make it const
Catch exceptions by const reference
Avoid Qt4 vs. Qt5 code duplication

Move ifdef into single function
Make const
Remove question before import

Do not ask if the user "wants to modify M and add N tasks" but just
import and show the result. I can't see any point in asking the user
without showing details. All this question creates is confusion.
Remove stale settings

If there's no value for a given key, clear it.
This avoids mixing old settings with new (incomplete) ones.
Do not assert that file exists

It's a bad idea to assert on things we can't control.
Also, this case is handled by the TaskExporter.
minor: isEmpty() instead of count() == 0
Avoid temporary file when importing downloaded tasks

Make it possible to import the downloaded XML data directly.
Remove define

Don't use a public macro for what can be a
private variable.
Use const ref.
Move summary calculation out of UI code