Feature/delete actions

This PR simply adds a *confirm* dialog to the buttons for deleting collections and decks. If the user selects OK, then the item will be deleted, otherwise the action is cancelled. It uses the built-in browser confirmation dialog, but this can be switched later to something prettier if desired.

This also fixes an issue on the deck view when there are no cards. the slider was throwing an unnecessary exception preventing the dialog handler from being activated.

@jazahn review?