qt: add qr reading from file to ScanQRTextEdit and SendTab

[f321x]

There was no ability to read qr codes contained in image files. This is confusing 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 getting loaded. Having the option to explicitly load a QR from file makes this clear.
Besides that it seems useful, considering reading QR from screenshots doesn't work on wayland.

[accumulator]

Nothing wrong with the code per se, but it doesn't find a QR code in my testing (screenshot saved as png from spectacle, which itself detects a QR code)

[f321x]

That's weird, works fine for me with the following screenshots:

Can you share a png that doesn't work?

[accumulator]

[accumulator]

That's weird, works fine for me with the following screenshots:

Yeah, your screenshots work fine here too

[f321x]

The zbar library we use here for detecting the qr codes doesn't seem to like dense qr codes that are a large portion of the image. So i added a function _reduce_qr_code_density() to create a new image of which the original image is only a smaller part (it pads the image with white space), so it can attempt the scan multiple times with the qr code being different proportions of the image. This seems to make it more reliable, also with your screenshot.