[feature] added draw background checkbox in composer items

Adds the ability to completely disable composer's items backgrounds.

This has the following benefits :
- more consistent with the way composer's items frames are treated
- allows workaround this bug with non-opaque elements http://hub.qgis.org/issues/6856

By the way, I reorganized the composer's items base widget, which wasn't very clear.

Note :
- I didn't do anything with SIP bindings or tests

![commitecompare](https://f.cloud.github.com/assets/1894106/105496/d563d946-69b3-11e2-9f6d-89f0a4161594.png)