Add OCR settings menu for choosing engine

Add a settings menu to the floating OCR button that opens a popup
window with an engine choice field.

The chosen engine is saved to a new 'wikisource-ocr' LocalStorage JSON value.

Also send the Content Language to the tool in the OCR request.

Bug: T281769
Change-Id: I5fe27ce29bada020160adb6564110178de5621b1