Add continuous panel feature for continous view mode

This PR adds a panel in the continuous view that continually show the short instrument name, clef, key signature and time signature.

To achieve this feature, 2 files were added for the new ContinuousPanel class.

A public updateContinuousPanel slot (and the update() line) were added to force the update of the panel when the user scrolls around the score or move it with the navigator.

![continuouspanel](https://cloud.githubusercontent.com/assets/5083355/3005696/0a94f514-ddee-11e3-96be-e2bf24fb4873.png)