Use flexible font size for QRCode image address (Jonas Schnelli)
Pull request description:
Bech32 addresses are currently cut off in the QRCode image in the GUI receive tab.
This adds a simple font size calculation logic that "must" (down to 4pt) fix into the given image width.
Examples OSX HiDPI:
<img width="332" alt="bildschirmfoto 2018-01-12 um 11 25 40" src="https://user-images.githubusercontent.com/178464/34896144-c0c65d76-f78c-11e7-93e1-94dc8e203269.png">
<img width="322" alt="bildschirmfoto 2018-01-12 um 11 25 46" src="https://user-images.githubusercontent.com/178464/34896145-c0edfe1c-f78c-11e7-8c09-c15155e2160e.png">
Examples Ubuntu non HIDPI:
<img width="314" alt="bildschirmfoto 2018-01-12 um 11 27 51" src="https://user-images.githubusercontent.com/178464/34896151-c88347f4-f78c-11e7-8a03-df8049dcfed6.png">
<img width="322" alt="bildschirmfoto 2018-01-12 um 11 27 42" src="https://user-images.githubusercontent.com/178464/34896152-c8bb881c-f78c-11e7-89d2-6f04ec608a19.png">
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Backport of Core PR12173