Factor out save current canvas view to image file code from symbol editor
code so it can be used anywhere.
Add ability to save to any image format supported by wxBitmapType. See
https://docs.wxwidgets.org/3.0/gdicmn_8h.html#a90a1eb6d85b5044a99b706fd979f27f5.
Currently only PNG output is implemented.
Please note that there is a minor bug that appears to be due to the
scroll bars which causes unfilled areas on the right and bottom edges
of the image. This always existed in the save symbol editor view
image but it was not as noticeable because by default the background
color is white. It is very noticeable in the footprint editor with
a black background.
The usual smattering of coding policy and comment fixes.
Fixes lp:1802127
https://bugs.launchpad.net/kicad/+bug/1802127
f1f4473d8b