Remove all cached data when changing the API key in module settings

Problem/Motivation

Pixabay search results are still cached, after changing the API key in module settings.

Steps to reproduce

• Install and configure the module as stated in the readme file
• Perform a search in the widget with a valid search key
• Change the API key in module settings
• Perform the same search with the same search key in the widget -> results will be shown
• Perform a search with a different valid search key (that you haven't done before) -> Only now the API key validation error is shown

Proposed resolution

Remove the search term from the cache tag, when cache data is set and invalidate the cache based on this new tag, when the API key is changed in module settings.