process-window: Add toolbar button to expand/collapse all process trees

Summary

Adds a toolbar button that toggles expand-all / collapse-all for process trees when "Show processes as a tree" is enabled.

• Visible only in tree mode (hidden for the flat process list).
• Expand mode restores the existing auto-expand-on-new-child behaviour.
• Collapse mode folds every group and keeps newly appeared children folded until the user expands again.
• Tooltip and icon update to reflect the next action (pan-down-symbolic / pan-end-symbolic).

This fills a usability gap when browsing deep process hierarchies: users no longer have to open/close each parent row by hand.

Test plan

• Enable Show processes as tree in Settings.
• Confirm the new toolbar button appears after Identify/Settings.
• Click once: all process groups collapse; tooltip becomes "Expand all process trees".
• Wait for process list refresh / spawn a short-lived child process: new children stay collapsed.
• Click again: all groups expand; new children auto-expand again.
• Disable tree mode: button hides; re-enable: button returns in expanded state.

AI assistance disclosure

This merge request was prepared with assistance from an AI coding agent (Cursor). I reviewed and take responsibility for every changed line.

Related docs: https://docs.xfce.org/apps/xfce4-taskmanager/4.20/start