theme_previewer changes.

[RoboPickle]

Overview

Allows the theme_previewer function to return the name of the selected theme.

Issue Fixes:

Fixes: #20

Example usage

See the issue #20 for a more comprehensive example

User can now choose a theme by clicking on the OK button of the previewed theme. The function returns the theme name.

e.g.

new_theme = sg.preview_all_look_and_feel_themes(scrollable=True,columns=8)
if new_theme:
    sg.theme(new_theme)

[spyoungtech]

Thanks for this contribution and for your patience in waiting for my review.