trimage builder specifies wrong attributes for altering image center

[RussBerg]

the lines
https://github.com/PierreRaybaut/guiqwt/blob/83bb8bc3ea67f5e8a2e3a54c142fe8597380dc50/guiqwt/builder.py#L1015-L1016

mistakenly try to set x0 and y0 of the trimage parameter except the attributes are actually pos_x0 and pos_y0, so the lines should be

pos_x0=x0

pos_y0=y0

otherwise the actual center of the trimage parameter (and therefore the image in the plot) will not be set if user specifies x0 and y0 values and instead use the default of both set to 0.0

I will submit a PR shortly

[RussBerg]

or would if I was added as a collaborator :)