Allow some tabbar to have alternative appearance if it's not tab

This allows non-tab items using "widgets/tabbar" to have alternative appearance (e.g. button-like) while leaving the real tab bar appearance as-is.

To enable this, theme authors need to add the following elements into the "widgets/tabbar" SVG:

• "normal"
• "hover"
• "pressed"
• "selected"
• "selected-hover"
• "selected-pressed"

Otherwise, those non-tab items will look the same as before.

Before	After (with a theme that supports this)