Stop passing around gc and pi

Make the graphics_context part of the ProfileGraphicsView and remember
that the plot info is already a part of the graphics_context (we kept
passing around both of them in the Gtk code... pointless but a leftover
from before adding the pi to the gc...)

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Add tooltip data and cleanup size calculations

Tomaz' code had a fixed height per tooltip item and some rather suspicious
logic how to position them (and how to size the surrounding box), based on
a fixed height in pixels per item - which of course fails if you use
larger fonts or multi line items.

This uses the bounding rects to correctly calculate the sizes and populates
the tooltip with the other dive data that we already had in a helper
function.

This also fixes a small formatting issue for gas change events.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>