tests: lib/i3test: Remove open_standard_window, introduce open_window

open_window has a better API than open_standard_window. It uses named
parameters and supplies default values for everything you don’t specify. This
way, you can use every feature which X11::XCB::Window supports.

testcases/t/65-for_window.t

@@ -186,7 +186,7 @@ wait_for_map $x;
 cmp_ok(@content, '==', 1, 'one node on this workspace now');
 is($content[0]->{border}, 'none', 'no border');
 
-my $other = open_standard_window($x);
+my $other = open_window($x);
 
 @content = @{get_ws_content($tmp)};
 cmp_ok(@content, '==', 2, 'two nodes');