Doc: Merge duplicated example directories

examples/quick/customitems contains all the examples found in
examples/quick/ui-components, plus 2 more (maskedmousearea and
painteditem). There are some very minor differences between the
duplicated files, regarding the "smooth" property. Apart from that, the
examples are identical.

The only file that is unique to examples/quick/ui-components is
example-slideswitch.qdoc. Moving it does not change the generated HTML
filename. The other examples do not produce any documentation.

Change-Id: I507d9064a60fd1f3a1469c1e423d4c0a72c7dc41
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>
Fix scenegraph backend API for material shaders.

Without this change, it is not possible to implement a custom renderer
(it has no way to compile or initialize material shaders).

Change-Id: Ie8778f739f3551f88f0d44ccb9769063a87ff276
Reviewed-by: Gunnar Sletta <gunnar.sletta@jollamobile.com>