1. Create a slide with multiple objects. Assume that we want to animate them.
2. We start by selecting the first object, and then clicking on the "Add..."
button, located in the "Modify Effect" section of the "Custom Animation" panel.

A "Add animation" window appears. We select the desired effect and press OK.
The effect is applied to the object, and the window closes automatically.

Here lies the problem: If there are multiple objects, we have to repeat step-2
for each object separately. Each time we have to start the same window.

This unnecessary labor can be avoided by making the "Add animation" window
modeless. Provide an extra button "Apply", which allows the user to apply the
effect to the selected object WITHOUT closing the window. 

The user should be able to select another object and apply an effect using the
same window.