Improvements in variables widget
Added info. panel to show it, when scene is empty
Merge pull request #747 from gorco/MultiplesVarImprovements

Improvements in variables widget
Now skins-raw aren't packed within the .apk file.
Now thumbnails are being unloaded/created/loaded whenever needed.
Adds ColorPicker and SlideColorPicker.
Improves PaintToolbar layout and colors.
Improves ElementContext layout and adds a ColorPicker for labels.
Model Label now has a color attribute.
Removes collider creation in Mockup for images.
Adds some asynchronous actions to open and close the game.
Adds paste icon to TopEditionToolbar and improves its layout.
Improves ElementContext behavior.
Now HiddenPanel doesn't cancel the event. Touchable actors are no longer needed.
Adds Toasts to MockupViews: an easier way to show notifications.
KeyboardFocus dissapears if we press ENTER in a textfield.
Improves some widgets and layouts. Also other minor fixes.