ipc: implement GET_VERSION to find out the i3 version

This is useful for third-party scripts which require certain features
and want to error out cleanly when they are run with an old i3 version.

Additionally, i3 --version might be different from what’s actually
running (an old version of the binary), so i3-msg -t get_version will be
the best way to figure out the i3 version you are actually running from
this commit on.

testcases/t/193-ipc-version.t

@@ -0,0 +1,23 @@
+#!perl
+# vim:ts=4:sw=4:expandtab
+# Verifies that we can get the version number of i3 via IPC.
+use i3test;
+
+my $i3 = i3(get_socket_path());
+$i3->connect->recv;
+# We explicitly send the version message because AnyEvent::I3’s 'version' sugar
+# method has a fallback which tries to parse the version number from i3
+# --version for older versions, and we want to avoid using that.
+my $version = $i3->message(7, "")->recv;
+
+# We need to change this when the major version changes (but we need to touch a
+# lot of changes then anyways).
+is($version->{major}, 4, 'major version is 4');
+
+cmp_ok($version->{minor}, '>', 0, 'minor version > 0');
+
+is(int($version->{minor}), $version->{minor}, 'minor version is an integer');
+is(int($version->{patch}), $version->{patch}, 'patch version is an integer');
+like($version->{human_readable}, qr/branch/, 'human readable version contains branch name');
+
+done_testing;