There was no ability to read qr codes contained in image files. This
could lead to confusion in some contexts, as `on_file_input()` of
ScanQRTextEdit will read the whole content of the file (instead of
looking for qr codes). The revealer plugin for example generates png
files containing qr codes and uses the `ScanQRTextEdit` to get user
input, for the user it would seem logical to click on 'Read from file'
to load the generated file, however this will result in the wrong data
being loaded. Having the option to explicitly load a QR from file makes
this clear. Also it seems useful, especially considering reading QR from
screenshots doesn't work on wayland.
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