add Hide/Show key actions

Usage example is to put this in your `wezterm.toml` to get the
macOS CMD-M shortcut for minimizing a window:

```
[[keys]]
key = "m"
mods = "CMD"
action = "Hide"
```

Refs: https://github.com/wez/wezterm/issues/32

src/config.rs

@@ -104,6 +104,8 @@ impl std::convert::TryInto<KeyAssignment> for &Key {
             KeyAction::ToggleFullScreen => KeyAssignment::ToggleFullScreen,
             KeyAction::Copy => KeyAssignment::Copy,
             KeyAction::Paste => KeyAssignment::Paste,
+            KeyAction::Hide => KeyAssignment::Hide,
+            KeyAction::Show => KeyAssignment::Show,
             KeyAction::IncreaseFontSize => KeyAssignment::IncreaseFontSize,
             KeyAction::DecreaseFontSize => KeyAssignment::DecreaseFontSize,
             KeyAction::ResetFontSize => KeyAssignment::ResetFontSize,
@@ -144,6 +146,8 @@ pub enum KeyAction {
     ActivateTab,
     SendString,
     Nop,
+    Hide,
+    Show,
 }
 
 fn de_keycode<'de, D>(deserializer: D) -> Result<KeyCode, D::Error>

src/frontend/glium/window.rs

@@ -101,6 +101,15 @@ impl TerminalWindow for GliumTerminalWindow {
         Ok(())
     }
 
+    fn hide_window(&mut self) {
+        eprintln!("Hide");
+        self.host.display.gl_window().hide();
+    }
+
+    fn show_window(&mut self) {
+        self.host.display.gl_window().show();
+    }
+
     fn frame(&self) -> glium::Frame {
         self.host.display.draw()
     }

src/frontend/guicommon/host.rs

@@ -29,6 +29,8 @@ pub enum KeyAssignment {
     ActivateTab(usize),
     SendString(String),
     Nop,
+    Hide,
+    Show,
 }
 
 pub trait HostHelper {
@@ -166,6 +168,8 @@ impl<H: HostHelper> HostImpl<H> {
             ResetFontSize => self.reset_font_size(),
             ActivateTab(n) => self.activate_tab(*n),
             SendString(s) => tab.writer().write_all(s.as_bytes())?,
+            Hide => self.hide_window(),
+            Show => self.show_window(),
             Nop => {}
         }
         Ok(())
@@ -269,6 +273,20 @@ impl<H: HostHelper> HostImpl<H> {
             win.scaling_changed(Some(1.0), None, dims.width, dims.height)
         })
     }
+
+    pub fn hide_window(&mut self) {
+        self.with_window(move |win| {
+            win.hide_window();
+            Ok(())
+        });
+    }
+
+    pub fn show_window(&mut self) {
+        self.with_window(move |win| {
+            win.show_window();
+            Ok(())
+        });
+    }
 }
 
 impl<H: HostHelper> Deref for HostImpl<H> {

src/frontend/guicommon/window.rs

@@ -45,6 +45,9 @@ pub trait TerminalWindow {
         Ok(())
     }
 
+    fn hide_window(&mut self) {}
+    fn show_window(&mut self) {}
+
     fn activate_tab(&mut self, tab_idx: usize) -> Result<(), Error> {
         let mux = Mux::get().unwrap();
         let mut window = mux