Added setIconFixedWidth() and isIconFixedWidth() to HasIcon.

- Added option to set fixed width to Icon which is part of another widget.
  Other Icon properties were already part of HasIcon interface.
  Issue is covered in #208.

Conflicts:
	core/src/main/java/com/dianaui/universal/core/client/ui/Anchor.java
	core/src/main/java/com/dianaui/universal/core/client/ui/InputGroupAddon.java
	core/src/main/java/com/dianaui/universal/core/client/ui/base/AbstractAnchorListItem.java
	core/src/main/java/com/dianaui/universal/core/client/ui/base/AbstractFormElement.java
	core/src/main/java/com/dianaui/universal/core/client/ui/base/HasIcon.java
	core/src/main/java/com/dianaui/universal/core/client/ui/base/button/AbstractIconButton.java
	gwtbootstrap3/src/main/java/org/gwtbootstrap3/client/ui/base/mixin/IconTextMixin.java