GtkMagnifier: Add a resizing mode

In addition to the fixed-size mode that is used by the text view,
add a mode in which the magnifier requests enough size to render
the entire inspected widget at the current magnification. In this
mode, the magnifier will update its size when the size of the
inspected widget changes. Also, make the magnifier redraw on its
own whenever the inspected widget draws.
GtkTextView: disconnect the magifier on destroy

This prevents warnings that would otherwise happen when
the magifier is finalized.
inspector: Add a magnifier

Add a magnifier that shows the selected widget up to 5 times
enlarged.
inspector: Remove an errant shadow