add ImageGenerator and UnCategorizedTemplateGenerator

ported from compat branch

APISite.uncategorizedtemplates() was available since
https://www.mediawiki.org/wiki/Special:Code/pywikipedia/9843
but had no corresponding pagegenerators

renamed UnCategorizedTemplatesGenerator
   into UnCategorizedTemplateGenerator
for consistency reasons, as:
* UnCategorizedCategoryGenerator
* UnCategorizedImageGenerator
* UnCategorizedPageGenerator

bug: 55007
Change-Id: I961310f6b3f13f1ac3880857e0f454576a0cbce6
Merge "add ImageGenerator and UnCategorizedTemplateGenerator"