feat: add toolbox text style support

[Ovilia]

#10202 Requested a more flexible toolbox label configuring method. This PR fulfills that requirement but the little triangle in the design of https://gitlab.com/gitlab-org/gitlab-ce/issues/57130 is not supported.

Requirement:

Example of what you can get from this PR:

The text color, background color, border radius, text padding can all be configured.

But the small triangle is currently not supported, because we have to add new option entry for that and thus needs further discussion. @jivanvl Please see if this meets your requirement.

---

In this PR, I added support for several options for toolbox.emphasis.iconStyle, namely:

• textPosition, textFill, textAlign: these options were supported but have no effect for settings in toolbox.emphasis.iconStyle.xxx due to a bug and they were fixed in this PR.
• textBackgroundColor, textBorderRadius, textPadding: these are newly supported options.

They can be set in toolbox.emphasis.iconStyle for all features, or toolbox.feature.XXX.emphasis.iconStyle for a specific feature XXX.

option = {
    toolbox: {
        show: true,
        feature: {
            saveAsImage: {
                show: true,
                title: 'Save as Image',
                emphasis: {
                    iconStyle: {
                        textFill: 'red'
                    }
                }
            },
            restore: {
                show: true,
                title: 'Restore'
            }
        }
        emphasis: {
            iconStyle: {
                textBackgroundColor: '#222',
                textFill: '#ddd',
                textBorderRadius: 3,
                textPadding: [5, 8],
                textPosition: 'top'
            }
        }
    },
    ...
}