Merge pull request #54 from ericksson/master

 [Fixes bug #1081030] Fix focus for resize canvas/image dialog fields.
[Fixes bug #1086678] Fix the name of the Canvas Pad and add an icon.
[Bug #1164222] Make the error dialog for opening files show more detail.

The error dialog for opening files is now the same as the one for
unhandled exceptions, and the dialog nows shows a stack trace for the
exception. That error dialog can now be reused in other contexts as well
by calling the PintaCore.Chrome.ShowErrorDialog method.
[Fixes bug #1164222] Fix crash with small images with the Pixel Grid on.

Fixed a potential crash in the Open Images pad when opening small images
with the Pixel Grid enabled. Also, the Pixel Grid is now disabled for
previews in the Open Images pad, since it looks terrible.