Ability to animate multiple selected elements

[jortega-consultant]

Feature Description

As many publishers use animations throughout their stories, we would like improvements made to support the ability to select multiple objects/layers and apply a uniform animation to the group.

The implementation of this feature should comply with the following cases:

• The user should be able to select multiple objects or layers in the Google Web Story editor.
• The user should be able to apply a uniform animation to the selected objects or layers as a group.
• The user should be able to preview the animation before applying it.
• The feature should be intuitive to use, and the user interface should clearly indicate when multiple objects or layers are selected.

Alternatives Considered

Right now, users must animate each individual element, when selecting a group of objects/layers, the animation capabilities are not available.

Additional Context

[swissspidy]

Previously: #4105, #12830

[swissspidy]

As @ayushnirwal pointed out, on the surface it would be relatively simple to show the animations panel for multi-selection. However, the whole panel system is laid out for modifying elements themselves, whereas animations are actually a property on the page object. So making that work requires a bit more effort than originally anticipated.