Add keepMounted flag to Modal/Dialog

**Is your feature request related to a problem? Please describe.**
Yes. It would be nice to persist the content sent to a modal/dialog even after it is closed to improve rendering performance.

**Describe the solution you'd like**
The default behavior of mounting the content when the modal/dialog is visible and unmounting it on close animation is great. However, I would like a way to keep the content mounted regardless if it is open. Two options could exist:
1. `after-first` - After opening for the first time, mount the content and keep keep it mounted for subsequent opens
2. `always` - Eagerly mount the content right away and again keep it mounted

Proposing new flag `mountingBehavior` on modal/dialog of type: `'open-only' | 'after-first' | 'always'` defaulted to `'open-only'`

**Describe alternatives you've considered**
I'm in the process of creating a custom dialog to accomplish this.

**Additional context**
[MUI](https://mui.com/material-ui/api/modal/#modal-prop-keepMounted) has a `keepMounted` flag for this.