three QtFred fixes

[Goober5000]

• Fix QtFRED root notifications when inserting nested operators

Only emit rootNodeFormulaChanged when the wrapped node is a formula
root. Inserting above a nested expression previously reported a child
as an event, goal, or cutscene root, causing assertions and out-of-bounds
accesses in the receiving dialog model.

• Give each QtFRED ship independently owned arrival and departure cues

Pass the tree model to the cue update handlers and serialize a separate
expression for each eligible ship. Sharing one formula across selected
ships caused double frees on subsequent edits and left other ships
referencing freed nodes when only one ship was changed.

This also avoids allocating unused expressions when cue updates are
disabled or no selected ships are eligible.

• Synchronize QtFRED item editability after SEXP node changes

Update native Qt editability alongside the shared model flags so
variable and container references cannot be edited as plain text.
Restore inline editing when a reference is replaced with ordinary data,
and preserve item flags when moving or copying branches.

Reject edit requests for protected items before setting the editing
state, preventing later programmatic changes from being treated as
user edits.