Qt5 /still/ refuses to encode the '+' character, and gives no rational for it.
Improve error messages

When reading/writing files or XML parsing fails, give more details.
Add Ctrl+Return shortcut to the event editor dialog, triggers OK button.

I tried to get this into Qt itself, it was refused, so let's do it where
it makes sense (KF5 will do it on KDE workspaces automatically, but this
is also useful on other platforms for dialogs such as this one, with a textedit)
Respect the G

The G is co-maintainer now, so give him the cred. Will
make him famous!
Create Task dialog: fix button order

Use dialog button box to get the correct Ok/Cancel order
depending on the platform.
Nitpick: "OK", not "Ok"
Connect to clicked() signal, not pressed()

clicked() is the signal to use for buttons, not
pressed().
Show week numbers in date pickers

That actually makes sense for every occurence of QDateEdit. Especially
when manually creating events inside the event editor window this is really helpful.

Note that this is disabled by default in QDateEdit (see usage of
QCalendarWidget::setVerticalHeaderFormat inside QCalendarPopup::verifyCalendarInstance()
inside qdatetimeedit.cpp). Not sure why, probably because the majority of users of
QDateEdit don't need that feature(?).
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