[Feature] #43 Print at scales defined in Lizmap plugin also when project is in 3857

When project is in CRS 3857 and at least one base layer (Google, etc.) is added,
 then the scales available for navigation are forced to be compatible with the baselayers
 ("ignoring" the ones defined in Lizmap plugin).
 The same scales are applied also for printing.
 As anyway as the baselayers such Google are not printed,
 then for printing it would be good to use anyway the scales defined in Lizmap
 plugin.