Refactor tree_next

- Makes `tree_next` not recursive.
- Adds `focus next|prev [sibling]` command. See (1.) and (2.) in
https://github.com/i3/i3/issues/2587#issuecomment-378505551 (Issue also
requests move command, not implemented here).
- Directional focus command now supports command criteria.

Wrapping is not implemented inside a floating container. This was also
true before the refactor so I am not changing it here.

testcases/t/101-focus.t

@@ -35,9 +35,13 @@ my $bottom = open_window;
 # end sleeping for half a second to make sure i3 reacted
 #
 sub focus_after {
-    my $msg = shift;
+    my ($msg, $win_id) = @_;
 
-    cmd $msg;
+    if (defined($win_id)) {
+        cmd "[id=$win_id] $msg";
+    } else {
+        cmd $msg;
+    }
     return $x->input_focus;
 }
 
@@ -50,6 +54,14 @@ is($focus, $mid->id, "Middle window focused");
 $focus = focus_after('focus up');
 is($focus, $top->id, "Top window focused");
 
+# Same using command criteria
+$focus = focus_after('focus up', $bottom->id);
+is($focus, $mid->id, "Middle window focused");
+
+cmd 'focus down';
+$focus = focus_after('focus up', $mid->id);
+is($focus, $top->id, "Top window focused");
+
 #####################################################################
 # Test focus wrapping
 #####################################################################

testcases/t/307-focus-next-prev.t

@@ -0,0 +1,72 @@
+#!perl
+# vim:ts=4:sw=4:expandtab
+#
+# Please read the following documents before working on tests:
+# • https://build.i3wm.org/docs/testsuite.html
+#   (or docs/testsuite)
+#
+# • https://build.i3wm.org/docs/lib-i3test.html
+#   (alternatively: perldoc ./testcases/lib/i3test.pm)
+#
+# • https://build.i3wm.org/docs/ipc.html
+#   (or docs/ipc)
+#
+# • http://onyxneon.com/books/modern_perl/modern_perl_a4.pdf
+#   (unless you are already familiar with Perl)
+#
+# Test focus next|prev
+# Ticket: #2587
+use i3test;
+
+cmp_tree(
+    msg => "cmd 'prev' selects leaf 1/2",
+    layout_before => 'S[a b] V[c d* T[e f g]]',
+    layout_after => 'S[a b] V[c* d T[e f g]]',
+    cb => sub {
+        cmd 'focus prev';
+    });
+
+cmp_tree(
+    msg => "cmd 'prev' selects leaf 2/2",
+    layout_before => 'S[a b] V[c* d T[e f g]]',
+    layout_after => 'S[a b*] V[c d T[e f g]]',
+    cb => sub {
+        # c* -> V -> b*
+        cmd 'focus parent, focus prev';
+    });
+
+cmp_tree(
+    msg => "cmd 'prev sibling' selects leaf again",
+    layout_before => 'S[a b] V[c d* T[e f g]]',
+    layout_after => 'S[a b] V[c* d T[e f g]]',
+    cb => sub {
+        cmd 'focus prev sibling';
+    });
+
+cmp_tree(
+    msg => "cmd 'next' selects leaf",
+    # Notice that g is the last to open before focus moves to d*
+    layout_before => 'S[a b] V[c d* T[e f g]]',
+    layout_after => 'S[a b] V[c d T[e f g*]]',
+    cb => sub {
+        cmd 'focus next';
+    });
+
+cmp_tree(
+    msg => "cmd 'next sibling' selects parent 1/2",
+    layout_before => 'S[a b] V[c d* T[e f g]]',
+    layout_after => 'S[a b] V[c d T*[e f g]]',
+    cb => sub {
+        cmd 'focus next sibling';
+    });
+
+cmp_tree(
+    msg => "cmd 'next sibling' selects parent 2/2",
+    layout_before => 'S[a b*] V[c d T[e f g]]',
+    layout_after => 'S[a b] V*[c d T[e f g]]',
+    cb => sub {
+        # b* -> S* -> V*
+        cmd 'focus parent, focus next sibling';
+    });
+
+done_testing;