Arrow are now PlacedSymbol, needed to fix some 
getters and setters ; serialization too.

Kept old arrow mode under the name "arrow1"

Zoom is centered on the mouse pointer when done
whit the mouse wheel

Added button for 100% and fitAll in the TB, fixed
css too

Warning:
pnikolov-bootstrap-colorpicker was modified
I should fork it and do the modifications in 
an other repo...

Functions for Thumbnail creation and borders are 
now in adim_app.utils

Thumbnail creation and border are called at view
level from adim_app (no more in models)

Thumbnail size in settings, no more hardcoded

Image and thumbnail URLs are now seen as
"sub pages" of the annotable 

image     .../<anobj_uuid>/image/
thumbnail .../<anobj_uuid>/thumb/