Move admin table theming from theme function to form's pre_render callback

There's one more use of the '#theme' => 'styleswitcher_admin_style_overview' in StyleswitcherAdmin::buildForm(), which is not worked out in the patch.

Reviewing.

I think the best would be to have a template instead of styleswitcher_admin_style_overview and a pre_render callback instead of theme_styleswitcher_admin_styles_table.

Automatically closed - issue fixed for 2 weeks with no activity.