Stroke: custom arrowheads from an arrowheads file join the arrowhead menus

The Stroke panel menu's Edit Arrowheads… (stroke.arrowheads.edit) opens
Arrowheads.vectorcraft next to the preferences, creating it with one
example head when it is missing. Each symbol in the file is one head,
listed by name after the built-in heads in both arrowhead menus: it
points right, its registration point is the tip, and its height is drawn
as wide as the built-in heads. The file is read at startup and again
each time VectorCraft saves it; stroke.arrowheads.reload and
stroke.arrowheads.list read and list it.

Arrowhead gets a Custom variant, appended after the built-in names, that
carries the head's name and outline, so a document that uses one draws,
saves, reopens and exports it without the arrowheads file. stroke.set
takes {"custom": name} or the {"Custom": {name, path}} form that
document.inspect reports.

Part of #5421.

crates/engine/src/lib.rs

@@ -151,6 +151,8 @@ pub struct Session {
     pub symbol_libraries: cmd::symbollib::Libraries,
     /// User Defined and loaded brush libraries (Window → Brush Libraries); not saved.
     pub brush_libraries: cmd::brushlib::Libraries,
+    /// The custom arrowheads of the arrowheads file (`stroke.arrowheads.*`); none without one.
+    pub arrowheads: cmd::arrowheads::Arrowheads,
     /// The Libraries panel's libraries of graphics, colours and text styles (`library.*`).
     pub libraries: cmd::library::Libraries,
     /// Where the saved PDF presets are kept, one file each; `None`: only for the session