Clipping groups

Groups just got a big upgrade! They now support:

• A “Clip content” option (this will clip the content outside of the group boundaries)
• A Corner Radius property
• A Color Fill property
• A Shadow property

Other improvements & fixes

• Fixed a rendering bug for groups with opacity < 100% or with shadows
• Improved the rendering engine to make the editing experience more fluid
• Added default values for Shadow and Background fields
• The timeline now crolls horizontally to show the animation selected last
• Improved the font rendering for a more consistent experience in the editor and in the exported files