Fixes two issues reported in #165:
- Export always produced a black background, making prints with a
white page wasteful/unreadable. Adds a Dark/White background choice
in the export modal, threaded through both PNG and SVG export.
- Firefox refused the download because the programmatic anchor was not
attached to the document. The anchor is now appended before click and
removed after, which Firefox requires.
Closes#165
ha-relevant: yes
Pouzor
fd2c50c1aa