Remember search text in Find/Replace in Files dialog

[miv391]

Fixes godotengine/godot-proposals#6340

Find/Replace in Files dialog works now using the same operating logic as vscode. If the user has selected something in the code editor, that selection is used. But if nothing is selected, then previous search string is not cleared. In Replace mode focus is set either to Find text edit or Replace text edit depending on what the user is likely to type next.

• Find in Files (Ctrl+Shift+F)

  • some text is selected -> use that as Find string
  • no text is selected -> do not change Find string

• Replace in Files (Ctrl+Shift+R)

  • some text is selected -> use that as Find string, focus Replace text edit
  • no text is selected -> do not change Find string, focus Find text edit

[akien-mga]

Thanks!