Auto merge of #454 - zed-industries:nsopenpanel, r=jdm

Add bindings for NSOpenPanel

This PR adds bindings for the main `NSOpenPanel` APIs, which let you present a modal for selecting files or directories. Usage:

```rust
let panel = NSOpenPanel::openPanel(nil);
panel.setCanChooseDirectories_(YES);
panel.setCanChooseFiles_(NO);
panel.setAllowsMultipleSelection_(YES);

let response = panel.runModal();
if response == NSModalResponse::NSModalResponseOk {
    let urls = panel.URLs();
    for i in 0..urls.count() {
        let url_ptr = urls.objectAtIndex(i).absoluteString().UTF8String();
        let url = CStr::from_ptr(url_ptr).to_string_lossy();
        eprintln!("You picked URL {}", url);
    }
}
```

Author: bors-servo

cocoa/src/appkit.rs

@@ -1774,6 +1774,52 @@ impl NSWindow for id {
     // TODO: Constraint-Based Layouts
 }
 
+#[repr(i64)]
+#[derive(Clone, Copy, Debug, PartialEq)]
+pub enum NSModalResponse {
+    NSModalResponseOk = 1,
+    NSModalResponseCancel = 0,
+}
+
+pub trait NSOpenPanel: Sized {
+    unsafe fn openPanel(_: Self) -> id {
+        msg_send![class!(NSOpenPanel), openPanel]
+    }
+
+    unsafe fn setCanChooseFiles_(self, canChooseFiles: BOOL);
+    unsafe fn setCanChooseDirectories_(self, canChooseDirectories: BOOL);
+    unsafe fn setResolvesAliases_(self, resolvesAliases: BOOL);
+    unsafe fn setAllowsMultipleSelection_(self, allowsMultipleSelection: BOOL);
+    unsafe fn URLs(self) -> id;
+    unsafe fn runModal(self) -> NSModalResponse;
+}
+
+impl NSOpenPanel for id {
+    unsafe fn setCanChooseFiles_(self, canChooseFiles: BOOL) {
+        msg_send![self, setCanChooseFiles: canChooseFiles]
+    }
+
+    unsafe fn setCanChooseDirectories_(self, canChooseDirectories: BOOL) {
+        msg_send![self, setCanChooseDirectories: canChooseDirectories]
+    }
+
+    unsafe fn setResolvesAliases_(self, resolvesAliases: BOOL) {
+        msg_send![self, setResolvesAliases: resolvesAliases]
+    }
+
+    unsafe fn setAllowsMultipleSelection_(self, allowsMultipleSelection: BOOL) {
+        msg_send![self, setAllowsMultipleSelection: allowsMultipleSelection]
+    }
+
+    unsafe fn URLs(self) -> id {
+        msg_send![self, URLs]
+    }
+
+    unsafe fn runModal(self) -> NSModalResponse {
+        msg_send![self, runModal]
+    }
+}
+
 #[repr(usize)]
 #[derive(Clone, Copy, Debug, Eq, PartialEq)]
 pub enum NSViewLayerContentsPlacement {