gtk: allow hiding window decoration in configuration

This is part of #319 by fixing it for GTK and introducing the
configuration option.

This adds `window-decoration = false` as a possible configuration
option. If set to `false`, then no window decorations are used.

src/apprt/gtk.zig

@@ -354,6 +354,12 @@ const Window = struct {
             c.gtk_widget_set_opacity(@ptrCast(window), app.config.@"background-opacity");
         }
 
+        // Hide window decoration if configured. This has to happen before
+        // `gtk_widget_show`.
+        if (!app.config.@"window-decoration") {
+            c.gtk_window_set_decorated(gtk_window, 0);
+        }
+
         c.gtk_widget_show(window);
         _ = c.g_signal_connect_data(window, "close-request", c.G_CALLBACK(&gtkCloseRequest), self, null, G_CONNECT_DEFAULT);
         _ = c.g_signal_connect_data(window, "destroy", c.G_CALLBACK(&gtkDestroy), self, null, G_CONNECT_DEFAULT);

src/config.zig

@@ -213,6 +213,11 @@ pub const Config = struct {
     /// specified in the configuration "font-size" will be used.
     @"window-inherit-font-size": bool = true,
 
+    /// If false, windows won't have native decorations, i.e. titlebar and
+    /// borders.
+    /// Currently only supported with GTK.
+    @"window-decoration": bool = true,
+
     /// Whether to allow programs running in the terminal to read/write to
     /// the system clipboard (OSC 52, for googling). The default is to
     /// disallow clipboard reading but allow writing.