Improve selection action toggle code

@broulik noticed an issue in the code. This commit fixes it.

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Before this commit there was a `QObject::disconnect` call that did
nothing (because it had `nullptr` as the first parameter) and there was a `QObject::connect` call that created
the same connections multiple times because of this.

This had no effect on end users. However such code can lead to
issues in the future e.g. if we ever had a situation in which the selection mode could be toggled for an inactive view container.

This commit solves this by replacing the `QObject::disconnect` call
with one that works. The `QObject::connect` call is moved so
there won't be multiple connections of the same type.