Set the maximum values for width and height to 100000000.000000

This is the same value as in the new image dialog, which makes
sense. This partially reverts

commit 40c93621
Author: Juan Palacios <jpalaciosdev@gmail.com>
Date:   Sun Nov 24 20:22:24 2013 +0100

    Tweak value range of size dialog's spinboxes.

    Incremented number of decimals for QDoubleSpinbox and set a more sane
    range for allowed values in all spinboxes of canvas size, image size
    and layer size dialogs.

BUG:391763
(cherry picked from commit 581d1379)