wip: Changes of the week

- Move engine to a different crate
- Add engine trait
- Refactor the rest of the codebase to work with these changes
- Add debug ui buffer, use it to finish imgui support

core/src/debug_ui.rs

@@ -0,0 +1,40 @@
+pub enum UICommand {
+    Text(String),
+    Separator,
+}
+
+pub struct DebugUIBuffer {
+    cmds: Vec<UICommand>,
+}
+
+impl DebugUIBuffer {
+    pub fn new() -> DebugUIBuffer {
+        DebugUIBuffer { cmds: vec![] }
+    }
+
+    // Commands
+    pub fn text(&mut self, text: String) {
+        self.cmds.push(UICommand::Text(text));
+    }
+
+    pub fn separator(&mut self) {
+        self.cmds.push(UICommand::Separator);
+    }
+
+    pub fn write_buffer(&self, ui: &imgui::Ui) {
+        for cmd in &self.cmds {
+            match cmd {
+                UICommand::Text(s) => {
+                    ui.text(s);
+                }
+                UICommand::Separator => {
+                    ui.separator();
+                }
+            }
+        }
+    }
+
+    pub fn reset_buffer(&mut self) {
+        self.cmds = vec![];
+    }
+}