New tab bar layout options

Improvements

• Add two new configuration options to MaterialTabBar that can be useful when implementing custom tab labels:
  • fillAvailableSpace: Applicable when tab labels don't inherently fill the width of the tab bar. When true (the default), the label widths are expanded proportinally to fill the tab bar. When false, the labels are not expanded and centered horizontally within the tab bar.
  • spacing: The amount of horizontal spacing to use between tab labels. Primary and Secondary tabs should use the default spacing of 0 to form a continuous line across the bottom of the tab bar.