Argument for 'kill' for killing a specific window (now default) or the whole client (+test)

Use 'kill window' to kill a specific window (for example only one specific
popup), use 'kill client' to kill the whole application (or X11 connection to
be specific).

testcases/t/64-kill-win-vs-client.t

@@ -0,0 +1,66 @@
+#!perl
+# vim:ts=4:sw=4:expandtab
+#
+# Tests if WM_STATE is WM_STATE_NORMAL when mapped and WM_STATE_WITHDRAWN when
+# unmapped.
+#
+use X11::XCB qw(:all);
+use X11::XCB::Connection;
+use i3test;
+
+my $x = X11::XCB::Connection->new;
+
+sub two_windows {
+    my $tmp = fresh_workspace;
+
+    ok(@{get_ws_content($tmp)} == 0, 'no containers yet');
+
+    my $first = open_standard_window($x);
+    my $second = open_standard_window($x);
+
+    is($x->input_focus, $second->id, 'second window focused');
+    ok(@{get_ws_content($tmp)} == 2, 'two containers opened');
+
+    return $tmp;
+}
+
+##############################################################
+# 1: open two windows (in the same client), kill one and see if
+# the other one is still there
+##############################################################
+
+my $tmp = two_windows;
+
+cmd 'kill';
+
+sleep 0.25;
+
+ok(@{get_ws_content($tmp)} == 1, 'one container left after killing');
+
+##############################################################
+# 2: same test case as test 1, but with the explicit variant
+# 'kill window'
+##############################################################
+
+my $tmp = two_windows;
+
+cmd 'kill window';
+
+sleep 0.25;
+
+ok(@{get_ws_content($tmp)} == 1, 'one container left after killing');
+
+##############################################################
+# 3: open two windows (in the same client), use 'kill client'
+# and check if both are gone
+##############################################################
+
+my $tmp = two_windows;
+
+cmd 'kill client';
+
+sleep 0.25;
+
+ok(@{get_ws_content($tmp)} == 0, 'no containers left after killing');
+
+done_testing;