Add resizable XML source panel below tree view

[rdmitry0911]

Summary

• Splits the main window with a vertical PanedWindow: the plist tree view occupies the top pane (~70%) and a new XML source panel sits below it
• Tree→Text sync: tree edits automatically refresh the XML panel with a 100 ms debounce (hooked into add_undo and reundo)
• Text→Tree sync: edits in the XML panel are applied to the tree after a 600 ms debounce, or immediately via the Apply button / Cmd+Return
• Search sync: find_next/find_prev now also highlight the matching element in the XML panel (yellow tag) and scroll to it
• Toggle: the panel can be shown/hidden with Cmd/Ctrl+B (also added to the Windows menu)
• Initial sash position set to 70% tree / 30% text on first open

Test plan

• Open a plist file and verify the XML panel appears below the tree
• Edit a node in the tree and confirm the XML panel updates (~100 ms delay)
• Edit XML directly in the panel and confirm the tree rebuilds (~600 ms delay)
• Click Apply or press Cmd/Ctrl+Return in the XML panel for immediate sync
• Use Find (Cmd/Ctrl+F) and navigate results — check that the XML panel scrolls to and highlights the match
• Toggle the panel with Cmd/Ctrl+B and verify it hides/shows
• Resize the sash and reopen the file to confirm layout behaviour

🤖 Generated with Claude Code