Fix#5321 field insertion over selected text in FormulaEditor

[leehaut]

Fix #5321

Summary

• Fix double-click field/function insertion in the expression editor so a selected range is replaced correctly regardless of selection direction (left-to-right vs right-to-left).
• Replace the caret/substring/setText logic with StyledText.insert(), which handles selection replacement properly and avoids wiping undo history.
• Apply the same fix to both User Defined Java Expression (janino) and Formula editor dialogs.

Root cause

The double-click handler treated getCaretPosition() as the start of the selection. When text is selected left-to-right, the caret is at the end of the selection, so the inserted field name was appended after the selection instead of replacing it.

Example:

• Expression: isOk? 1: 0
• Select 1 left → right, double-click field name → isOk? 1name: 0 (wrong)
• Select 1 right → left, double-click field name → isOk? name: 0 (correct)