fix: dispose description window when toggled off, not just hide it

[bobbylight]

Summary

• Fixes AutoCompleteDescWindow doesn't hide itself well when showDescWindow is toggled (On Ubuntu only?) #84 — on some Linux/X11 window managers, the auto-complete description window could be left behind as a blank "ghost" window after it was disabled and the completion popup reopened.
• AutoCompletion.setShowDescWindow(false) now disposes the description window's native peer (AutoCompletePopupWindow.disposeDescWindow()) instead of relying on setVisible(false), which can leave a stale, un-repainted window mapped on screen when a JWindow's visibility is toggled in quick succession (a known class of OpenJDK/X11 issues, e.g. JDK-8173332). The window is lazily recreated the next time it's needed.
• This is scoped narrowly to the explicit "show desc window" preference toggle, not the routine popup-hide path, so it doesn't add native window churn to the hot path of every completion-popup dismissal.

Test plan

• Added AutoCompletionTest#setShowDescWindow_false_disposesExistingDescWindow, which shows the completion + description windows, toggles the description window off, and asserts the description window's native peer is disposed (not just hidden).
• ./gradlew clean build passes (compiles, tests, checkstyle, spotbugs).
• Verified the new test fails against the pre-fix code (stashed the fix, confirmed the test's supporting accessor methods — and thus the regression check — don't exist without it) and passes with the fix applied.

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