Allow for theme independent alternating background color for TabularEditor

[joshduran]

Currently, the TabularEditor has an even_bg_color attribute that can be set to provide a visual contrast between alternating rows. This is very nice, but setting this attribute to a fixed color will not look good for both dark and light themed GUIs. It would be nice if we could instead set a flag (e.g. alternate_bg_color = True). Setting this flag would automatically set the even_bg_color to a slightly contrasting color considering the theme background color. This would not break any current code that uses the existing syntax while also simplifying code for those who want alternating background without needing precise control over the exact background color chosen.