Tablets have not always an eraser (most of the generic tablets like Huion,
UC-Logic, etc... don't). We should not reject such tablets.
Commit 54849a9 (wacom: Only the stylus and eraser tools need to exist)
mentioned that we were not sure about eraser, and I think we should not
assume one either.
To do so, we simply ignore the eraser xinput node and rely on
libwacom to actually provide the eraser information.
If the stylus does not have the eraser tip, we may fall in the
LAYOUT_OTHER case. We have a picture of a generic Wacom pen with an
eraser, and the leaders linking the widget to the picture are scrambled.
To prevent that, gray out the eraser pressure slider so that we do not
break the layout.
https://bugzilla.gnome.org/show_bug.cgi?id=746117
The airbrush tool has only one button, and the button mapping combo box
does not work.
We should not bail out if the widget has been removed, that means that
the stylus does not support the second button.
Check upfront the number of buttons to know if the top button parameter
is available.
https://bugzilla.gnome.org/show_bug.cgi?id=746263
When we just hide items inside a GtkGrid, they still take space
in the layout. So we need to remove those widgets, and change
the location in the grid of those widgets.
Finally, as some widgets are removed. the layout doesn't expand as
it used to, so make the rows homogeneous to line up with the drawings.
