This commit ports the work of EchterAgo and cculianu from Electron-Cash,
to implement a new toolchain to scan qr codes.
Previously, on Linux and Win, we have been using zbar to access the camera
and read qrcodes; and on macOS we used CalinsQRReader (an objective-C
project by cculianu).
The new toolchain added here can use QtMultimedia to access the camera,
and then feed that image into zbar. When used this way, zbar needs
fewer dependencies and is easier to compile, in particular it can be
compiled for macOS.
The new toolchain works on all three platforms, with some caveats
(see code comments in related commits) -- so we also keep the end-to-end
zbar toolchain; but at least we can drop CalinsQRReader.
The related changes in Electron-Cash are spread over 50+ commits (several PRs and direct
pushes to master), but see in particular:
https://github.com/Electron-Cash/Electron-Cash/pull/1376
some other interesting links:
b2b737001c163224cf1f3b31e0fcb1eda015908ehttps://github.com/Electron-Cash/Electron-Cash/pull/1545052aa06c23
Sometimes we want its "remember path" behaviour but it does not make sense to
parent the dialog from main window. When so, caller code no longer needs to
get a reference to a main window.
Also rm last usages of get_parent_main_window().
The few other cases that used SimpleConfig.get_instance() now
either get passed a config instance, or they try to get a reference
to something else that has a reference to a config.
(see lnsweep, qt/qrcodewidget, qt/qrtextedit)
