Add keyboard-searchable code picker for text coding

[willowsipling]

Closes #1493

Status

Draft for discussion toward QualCoder 4.1.

This implementation began as a working prototype for the workflow discussed in #1493. This is a draft per suggestions for potential merging into the 4.1.

Feature

Adds a keyboard-first searchable code picker in Code Text:

• In the Code Text editor view, user selects text, then presses F
• A new dialogue box appears, with a search text field, which is in focus automatically
• In the same dialogue box, underneath the search search text field, is a box in which all codes are displayed
• The code box is filtered live as a user types in the search text field, searching for their intended code
• Up/down arrows change the highlighted returned code, while typing focus remains in the search text field
• Enter applies the highlighted code to the text, and also closes the dialogue box
• Escape cancels the dialogue box

This was most-directly modeled on Zotero's citation picker, but, it's similar to most command palettes or the Spotlight search on MacOS.

Testing

Tested locally on macOS while running QualCoder from source:

• Dialogue box activation
• Selector opening only when text is selected
• Does live filtering correctly
• Arrow key navigation while maintaining search focus (aka, you don't have to re-select the search text box if you use the arrow keys to navigate through the list of returned codes)
• Continued typing after navigation
• Single assignment on enter, after which auto-closed the dialogue box
• Cancellation with Esc
• No action when no text is selected

Future Directions

• Could play more nicely with Recent Codes? E.g, maintain the GUI (instead of dialogue box) of Recent Codes? Or, perhaps recently-used codes could be what's auto-populated when a user first presses F?
• This is a skill issue on my part, but I don't understand the localization/translation functions, so this would need to be implemented. I am reading some Python books to learn more.