#14: Create feature styler

[joergreichert]

(https://github.com/Polymap4/polymap4-p4/issues/issue/14)

introduced

• Spinner
• TabFolder (default TabFolder cannot be used as its tab headers cannot be widen (easily), as MD spec (http://www.google.com/design/spec/components/tabs.html#) would require) factory methods
• font form field
• color form field
• enableable form field (form field container with checkbox)
• icon chooser form field (multi tab gallery with asynchronously loading images)

Handled some performance issues in SVG generator.

This pull request belongs to https://github.com/Polymap4/polymap4-p4/pull/15 and requires https://github.com/Polymap4/polymap4-core/pull/23