Add configuration option for disabling mouse warping

This patch adds a new configuration option "mouse_warping [output|none]".

When mouse warping is disabled, mouse cursor does not jump to middle of current
screen when changing workspaces between multiple outputs. This introduces a
"special" cursor state, where focus is in one window and cursor on another.
Useful for eg. scrolling a web page with mouse wheel while typing into another
window on keyboard.

testcases/t/519-mouse-warping.t

@@ -0,0 +1,52 @@
+#!perl
+# vim:ts=4:sw=4:expandtab
+#
+# Please read the following documents before working on tests:
+# • http://build.i3wm.org/docs/testsuite.html
+#   (or docs/testsuite)
+#
+# • http://build.i3wm.org/docs/lib-i3test.html
+#   (alternatively: perldoc ./testcases/lib/i3test.pm)
+#
+# • http://onyxneon.com/books/modern_perl/modern_perl_a4.pdf
+#   (unless you are already familiar with Perl)
+
+use i3test i3_autostart => 0;
+
+# Ensure the pointer is at (0, 0) so that we really start on the first
+# (the left) workspace.
+$x->root->warp_pointer(0, 0);
+
+my $config = <<EOT;
+# i3 config file (v4)
+font -misc-fixed-medium-r-normal--13-120-75-75-C-70-iso10646-1
+fake-outputs 1024x768+0+0,1024x768+1024+0
+mouse_warping none
+EOT
+my $pid = launch_with_config($config);
+
+my $i3 = i3(get_socket_path());
+
+######################################################
+# Open one workspace with one window on both outputs #
+######################################################
+
+# Open window on workspace 1, left output
+is(focused_ws, '1', 'starting with focus on workspace 1');
+open_window;
+
+# Open window on workspace 2, right output
+cmd 'focus output right';
+is(focused_ws, '2', 'moved focus to workspace 2');
+open_window;
+
+# If mouse_warping is disabled, the pointer has not moved from
+# position (0, 0) when focus was switched to workspace 2
+$x->root->warp_pointer(0, 0);
+
+# Ensure focus is still on workspace 2
+is(focused_ws, '2', 'warped mouse cursor to (0, 0), focus still in workspace 2');
+
+# Exit gracefully
+exit_gracefully($pid);
+done_testing;