Added webp image format plugin

Also added auto test and config test.

This plugin was disabled on winrt and android.

[ChangeLog] Added webp image format plugin.

Task-number: QTBUG-14205

Done-with: Liang Qi <liang.qi@digia.com>
Change-Id: I9409e894d7fc1aef9dce4ffdff8bdf483d689774
Reviewed-by: Ivan Komissarov <ABBAPOH@gmail.com>
Reviewed-by: Lars Knoll <lars.knoll@digia.com>