graph: Redesign nodes and ports

Nodes now have a background using the libadwaita .card style class.

Ports now have a circular handle, which is positioned on the edge of the node so that half of the circle sticks out.
Ports are also no longer themed like a button and don't receive a color based on the guessed media type, in a future commit,
the handle will be colored instead.

src/ui/graph/port_handle.rs

@@ -0,0 +1,84 @@
+// Copyright 2021 Tom A. Wagner <tom.a.wagner@protonmail.com>
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License version 3 as published by
+// the Free Software Foundation.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+//
+// SPDX-License-Identifier: GPL-3.0-only
+
+use adw::{glib, gtk, prelude::*, subclass::prelude::*};
+
+mod imp {
+    use super::*;
+
+    #[derive(Default)]
+    pub struct PortHandle {}
+
+    #[glib::object_subclass]
+    impl ObjectSubclass for PortHandle {
+        const NAME: &'static str = "HelvumPortHandle";
+        type Type = super::PortHandle;
+        type ParentType = gtk::Widget;
+
+        fn class_init(klass: &mut Self::Class) {
+            klass.set_css_name("port-handle");
+        }
+    }
+
+    impl ObjectImpl for PortHandle {
+        fn constructed(&self) {
+            self.parent_constructed();
+
+            let obj = &*self.obj();
+
+            obj.set_halign(gtk::Align::Center);
+            obj.set_valign(gtk::Align::Center);
+        }
+    }
+
+    impl WidgetImpl for PortHandle {
+        fn request_mode(&self) -> gtk::SizeRequestMode {
+            gtk::SizeRequestMode::ConstantSize
+        }
+
+        fn measure(&self, _orientation: gtk::Orientation, _for_size: i32) -> (i32, i32, i32, i32) {
+            (Self::HANDLE_SIZE, Self::HANDLE_SIZE, -1, -1)
+        }
+    }
+
+    impl PortHandle {
+        pub const HANDLE_SIZE: i32 = 14;
+    }
+}
+
+glib::wrapper! {
+    pub struct PortHandle(ObjectSubclass<imp::PortHandle>)
+        @extends gtk::Widget;
+}
+
+impl PortHandle {
+    pub fn new() -> Self {
+        glib::Object::new()
+    }
+
+    pub fn get_link_anchor(&self) -> gtk::graphene::Point {
+        gtk::graphene::Point::new(
+            imp::PortHandle::HANDLE_SIZE as f32 / 2.0,
+            imp::PortHandle::HANDLE_SIZE as f32 / 2.0,
+        )
+    }
+}
+
+impl Default for PortHandle {
+    fn default() -> Self {
+        Self::new()
+    }
+}