implement explicit Copy keybinding action

previously we would only ever copy to the clipboard when the selection
was changed.

Now we respect the Copy keypress and have it re-copy the selection
into the clipboard.

src/frontend/gui/termwindow.rs

@@ -646,7 +646,7 @@ impl TermWindow {
                 // self.toggle_full_screen(),
             }
             Copy => {
-                // Nominally copy, but that is implicit, so NOP
+                self.clipboard.set_contents(tab.selection_text())?;
             }
             Paste => {
                 tab.trickle_paste(self.clipboard.get_contents()?)?;

src/localtab.rs

@@ -93,6 +93,11 @@ impl Tab for LocalTab {
             .selection_range()
             .map(|r| r.clip_to_viewport(terminal.get_viewport_offset(), rows))
     }
+
+    fn selection_text(&self) -> Option<String> {
+        let terminal = self.terminal.borrow();
+        Some(terminal.get_selection_text())
+    }
 }
 
 impl LocalTab {

src/mux/tab.rs

@@ -67,6 +67,7 @@ pub trait Tab: Downcast {
     /// (eg: it has been normalized and had clip_to_viewport called
     /// on it prior to being returned)
     fn selection_range(&self) -> Option<SelectionRange>;
+    fn selection_text(&self) -> Option<String>;
 
     fn trickle_paste(&self, text: String) -> Fallible<()> {
         if text.len() <= PASTE_CHUNK_SIZE {

src/server/tab.rs

@@ -312,6 +312,11 @@ impl Tab for ClientTab {
             .lock()
             .unwrap()
     }
+
+    fn selection_text(&self) -> Option<String> {
+        // FIXME: get selection from peer or the surface
+        None
+    }
 }
 
 struct RenderableInner {

src/termwiztermtab.rs

@@ -294,6 +294,11 @@ impl Tab for TermWizTerminalTab {
             .lock()
             .unwrap()
     }
+
+    fn selection_text(&self) -> Option<String> {
+        // FIXME: grab it from the surface
+        None
+    }
 }
 
 pub struct TermWizTerminal {