Warning if 'allowfullscreen' config option has not been saved yet.

[donquixote]

In some places we query the new 'allowfullscreen' config option.
This causes a warning, if the new settings have not been saved yet.

Warning: Undefined array key "allowfullscreen"

[donquixote]

Note: This is mostly relevant for new settings, which have been introduced after the module was first installed on a website.

There are different ways to deal with this.
E.g. we could populate the setting with an update hook, but I would prefer to go for a simpler option, which can be improved later.