Update of composer item's GUI

Hi !

I've been working a little bit on the composer GUI (mainly ComposerItem's widgets).

### The differences ###

#### QToolBox replaced by QCollapsibleGroupBox(es). ####

*before*
- a lot of wasted space
- unpractical (you must toggle back and forth to edit elements on different pages)
- not very good looking (it looks messy and it's hard to distinguigh from QDockWindowss)

*after*
- QgsCollapsibleGroupBoxes look great and provide a nice outline of all features when folded
- Easier to adapt to your needs and screen size


#### QToolBox replaced by QCollapsibleGroupBox(es). ####

- so you can dynamically reposition/resize the items without having to open a modal dialog.
- also provides a nice feedback on element's position


#### Harmonisation and optimisation of widget layouts ####

- most of the widgets had uneven spacing (looked a bit sketchy)
- some widgets where uselessly big
- better hiearchy and grouping of similar options


#### Duplicate/Ambigious/Unclear functionalities ####

- QgsComposerShape had duplicate implemtation of frame and background


### Changes ###

The work was mainly done in the .ui files, but some classes are a little more modified :
- QgsComposerShape (clean duplicate frame/background implementation)
- QgsItemPositionDialog was merged into QgsComposerItemWidget, which also makes some change

### Screenshots ###

Here are screenshots of all modified widgets :
https://www.dropbox.com/sh/gkte3o126doe4ue/YfN_iVxRKx

### Todo ###

- I think it would be nice to integrate the atlas' widget into the composition's widget
- It would be nice if there was a collapse/expand all button (or a shortcut (alt-click ?) to collapse all other QgsCollapsibleBoxes)
- Use color buttons wherever possible

### So... ###

What do you think ?