config: add [csd].hide-when-maximized=yes|no

When enabled, the CSD titlebar will be hidden when the window is
maximized.

Closes #1019

tests/test-config.c

@@ -666,6 +666,8 @@ test_section_csd(void)
                &conf.csd.color.maximize);
     test_color(&ctx, &parse_section_csd, "button-close-color", true,
                &conf.csd.color.quit);
+    test_boolean(&ctx, &parse_section_csd, "hide-when-maximized",
+                 &conf.csd.hide_when_maximized);
 
     /* TODO: verify the ‘set’ bit is actually set for colors */
     /* TODO: font */