ToolsPanel: move away from hardcoded classnames

[ciampo]

What

Requires #34345 first, to make sure that tests don't rely on classnames.

We should refactor the ToolsPanel component so that it avoids using hardcoded classnames as much as possible.

Currently, the hardcoded classnames in TooolsPanel are:

• components-tools-panel. This class could be replaced by using the context system, so that ToolsPanelHeader knows if it's being rendered inside a ToolsPanel
• single-column. This class should be removed because it currently introduces circular dependencies with the block-editor and edit-site (as also specified in Block Supports: Switch dimensions inspector controls slot to bubble virtually #34725 (review))
• components-base-control as explained in Block Supports: Switch dimensions inspector controls slot to bubble virtually #34725 (comment)
• components-dropdown-menu and components-dropdown-menu__toggle — these classnames are going to be tougher to remove, since they would need first the DropdownMenu component to be refactored and use the new context System. After that, we could use the component's computed selector at runtime (example)

Why

We should avoid using hardcoded classnames as much as possible in our components:

• it goes against the Styling guidelines, where classnames are generated at runtime
• hardcoded classnames can be effectively treated as public API, which can later need to involuntary breaking changes later on.

A/C

• Remove the components-tools-panel classname
• Remove the single-column classname
• Remove the components-dropdown-menu and components-dropdown-menu__toggle classnames

[aaronrobertshaw]

A PR removing all the hardcoded classnames except for single-column is up: #35415

Adopting the Grid component to handle the ToolsPanel layout, and thus remove the single-column class, should be possible but will involve a little more work. I'll take care of that in a separate PR.