More alignments

Added three new alignments:

• center: Centers subviews horizontally within a row. Rows are centered within the container.
• centerDistribute: Extra space is first added equally before the first and after the last subviews up to the spacing amount, then distributed evenly between and around the subviews.
• centerJustify: Aligns the horizontal centers of subviews within a row. The first subview aligns with the leading edge and the last subview aligns with the trailing edge of the container. Extra space is distributed evenly between subviews. This is similar to how justified text is laid out in a paragraph.