Allow multiple panels open at once

[devinhalladay]

As a part of #1 we need the ability for users to open a separate category panel (eg. Names) from another panel (eg. Component Variables). This would allow users to temporarily set the state of their chosen category (eg. Names) in the context of their primary open panel (eg. Component Variables)

1. Open Component Variables panel
2. Select a layer and select a category for its placeholder text. Let's say I want to fill my text layer with a Time.
3. Click a button to open the options for that category. This would open the Time panel and allow me to configure its options.
4. Back out of the options and click Generate to fill the selected layer with a Time based on the options I configured.

The state updates to category options here should be "temporary" or scoped to the panel that is currently open.