Add `addClass` and `removeClass` methods to the block to deal with CSS classes

[BeksOmega]

To Fix

1. Add an addClass method to the BlockSvg class, that takes in a passed string, and calls dom.addClass with the this.svgGroup_ to add it as a CSS class on the block.
2. Add a removeClass method to the BlockSvg class, that takes in a passed string, and calls dom.removeClassClass with the this.svgGroup_.
3. Refactor any calls in the BlockSvg class to dom.addClass with this.svgGroup_ to call this.addClass instead.
4. Refactor any calls in the BlockSvg class to dom.removeClass with this.svgGroup_ to call this.removeClass instead.

The PR to fix this should be made against the rc/v12.0.0 branch.

To work on this issue, please comment below asking to be assigned.

[ishon19]

Hi @BeksOmega! I'd love to work on this and the dependent issue #8271 as well, can you please assign me to both of these whenever you get time? Thanks!

[BeksOmega]

Heya @ishon19 Happily =) But you have to comment on #8271 for me to assign you :/ (this is a restriction with the GitHub UI)

[ishon19]

Heya @ishon19 Happily =) But you have to comment on #8271 for me to assign you :/ (this is a restriction with the GitHub UI)

My bad! No problem, adding a comment over there.

[BeksOmega]

Closed by #8337