View: Optimize header pane sizing

Before, the header pane didn't resize to accomodate all signals
when a new session was created and a driver selected (e.g. demo).
To fix this, the header pane autosizing was put in a method of
its own and is now called when new signals are added.

Also, the name of header_fully_visible() needed adjustment
because it also returned true when it *wasn't* 100% visible
due to the margin of error that we permit. It's now called
header_was_shrunk() and its return value logic inverted.