[Border-image generator] `border-image-slice` `fill` keyword not toggled in CSS output

[strarsis]

Although the border-image-slice fill keyword is correctly toggled for the actual inline styles applied to the example element,
the CSS output (that can be viewed and copied by the user) is not. border-image-slice always contains the fill keyword,
regardless of the toggle state of the fill option.

[yarusome]

The fill value in the output CSS code is updated after following steps:

1. The "fill" option in the "border-image-slice" section is toggled;
2. Any input slider or dropdown menu in this section is updated, or any config button in this section is clicked (not necessarily updating values in the config panel).

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The script uses a lot of pre-ES6 tricks, which are beyond my knowledge to fix this issue.