Add Generator to share dialog

[udenr]

Add an option so one can for example select text in a different app and then share it with the qr code generator to generate a qr code. Mabe even the option to share other types, e.g. E-mail, contact, ...

[chrysn]

An app to possibly draw inspiration from here is the discontinued app at https://github.com/zxing/zxing, whose "share" feature worked well enough for me.