Doc: Fixed qmake manual's images and snippets directories.

-corrected qmake.qdocconf file to not read the manual twice
-moved the snippets and images directories to make sure they are not
 in the src directory

Task-number: QTBUG-32580
Change-Id: I7b702ce3b1c8709bbb1f473f70625083242f1a40
Reviewed-by: Martin Smith <martin.smith@digia.com>
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@digia.com>

qmake/doc/qmake.qdocconf

@@ -22,8 +22,8 @@ language                = Cpp
 
 sources                 = src/qmake-manual.qdoc
 
-imagedirs               = src/images
-exampledirs             = src
+imagedirs               = images
+exampledirs             = snippets
 
 depends += \
     qtwinextras