Make QQuickControl public

This would allow users to add new logic to controls, without committing to making the entire templates API public.

It would require exporting some API:

<jpnurmi> micurtis: actually just export macros in qqc2 to make it possible to call the begin/cancel/complete methods in qquickdeferredexecute.h from outside of libqtquicktemplates2
<jpnurmi> micurtis: as it is today, when building a custom control, if you follow the structure of any built-in control, you'll get linker errors