Fix dismiss button in selection mode paste bar

This one liner changes the signal that is emitted when the
"Dismiss" button is pressed on the Paste bottom bar in selection
mode. Before this commit, the `leaveSelectionModeRequested()`
signal was used but it had no effect because technically the
selection mode isn't active anymore when the paste bar is shown.

Everything works as expected if instead
`barVisibilityChangeRequested()` is emitted.