effects/overview: Add window filtering

This adds window filtering to the Overview effect. This satisfies both
Overview users' needs and the workflow of Windowview: When the user
starts typing, windows are filtered by default. If there is no matching
window, then the Milou UI is shown, including the option for opening a
new app is shown--the same one that currently gets shown when the user
starts typing. This leads to a very seamless experience, where the user
can type any app name and they will get it, no matter if it's already
opened or not.