Implement canvas switching by clicking on a canvas "tab" at the bottom

Implement canvas switching by clicking on a canvas "tab" at the bottom
of the main window.

Double-click on a canvas "tab" opens up a canvas-rename dialog box.

Remove canvas switching / selection entries from the top-level canvas menu.

Add several new key bindings: Home/End switches to first/last canvas,
cursor keys scroll/pan the current viewport on the current canvas.

Bug found by:
Submitted by:
Requested by:
Reviewed by:
Approved by:
Obtained from: