✨ Rename Overlay to SearchPanel

[austincondiff]

Description

Upon closer inspection, it looks like the following UI elements in Xcode all use the same view called IDESourcePanel.

• Library
• Open Quickly
• Quick Actions

We should rename relating items in CodeEdit to be consistent with Apple's naming conventions in Xcode.

The following changes should be made:

• OverlayPanel becomes SearchPanel
• OverlayView becomes SearchPanelView

[dklaver15]

@austincondiff I'd like to work on this.