Add camera icon to header bar for SVG printing?

[simonrharris]

At the moment pressing p is the only way to print to svg but it's not mentioned in the documentation. It would also be nice to have a button you could press to take a snapshot of the screen

[jameshadfield]

it is mentioned, in bold, near the top of the about page

but we can put a button somewhere too

[simonrharris]

I blame Izzy. She was complaining that it wasn't mentioned anywhere! Button would be nice though