linux: Primary clipboard (#10534)

Implements copying from and pasting to the primary selection.

Release Notes:

- N/A

crates/gpui/src/app.rs

@@ -547,11 +547,23 @@ impl AppContext {
         self.platform.window_appearance()
     }
 
+    /// Writes data to the primary selection buffer.
+    /// Only available on Linux.
+    pub fn write_to_primary(&self, item: ClipboardItem) {
+        self.platform.write_to_primary(item)
+    }
+
     /// Writes data to the platform clipboard.
     pub fn write_to_clipboard(&self, item: ClipboardItem) {
         self.platform.write_to_clipboard(item)
     }
 
+    /// Reads data from the primary selection buffer.
+    /// Only available on Linux.
+    pub fn read_from_primary(&self) -> Option<ClipboardItem> {
+        self.platform.read_from_primary()
+    }
+
     /// Reads data from the platform clipboard.
     pub fn read_from_clipboard(&self) -> Option<ClipboardItem> {
         self.platform.read_from_clipboard()