WIP: Fixes #132 - Added backup export button

I was trying to investigate the effort behind reintroducing the backup functionality in the UI and the export functionality actually turned out to be quite easy to implement. The import part, on the other hand, is a bit trickier because it requires further UI for error handling and to get user confirmation.

Therefore we could already move ahead with the exporter if we want. The implementation I chose has the following characteristics:

• Adds a "Backup" section to the option page's Advanced tab with a title, a description and a "Save backup" button
• Clicking on the button will create a patterns.ini file containing all filters and filter lists (same as the legacy backup feature) and initiate its download