Persistent quick-find

[supersaiyansubtlety]

Adds a "Persistent" checkbox and a "Close" button to QuickFindDialog.

The checkbox makes it so the dialog is not hidden when it loses focus.

Suggestions for a better label than "Persistent" are welcome.

To actually use this in Enigma, we'll need to:

• save the persistent option to disk
• replace the "Close" text with an X icon, probably the one used by Enigma's tabs
• either hide the dialogue when switching tabs, or attach a different dialogue to each tab

[supersaiyansubtlety]

superseded by #6 and QuiltMC/enigma#304