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/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.