filechooserwidget: Accept entered path in SELECT_FOLDER mode

In GTK 3 there was a bug causing the location entry to be cleared when changed, as
explained in the commit message of c99935a5

But not reacting at all to changes in the entry, means that the default action button
in GtkFileChooserDialog doesn't become sensitive when a valid folder path is entered.

This can be resolved by removing the condition introduced by the commit mentioned
earlier, and doing so doesn't reintroduce the bug that the condition used to avoid.

Fixes #4854
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