Slide preview is working again.

The problem is the size of these drawing area which is 1 by default. I set them
manualy to a value so they are visible, but if you resize the window, they don't
resize. It needs fixing.
Fix "goto with label" functionnality.

We can again use the current page label to navigate in the pdf.
'with Gdk.lock()' seems deprecated and unneeded.
Error on my part. Gdk is not deprecated, only some of its functions are.