i3bar: Group child processes for signalling

Set the process group id of the child process by calling `setpgid` after
forking and before calling `exec`.

The process group ID will be set to the process ID of the forked
process. Processes spawned by this child process will also have this
group ID.

Send signals to the process group with `killpg`. This will send the
signal to all of the process group.

fixes #1128
Refactor and improve test 514

Split test 514's assertion into three assertions to make it more
explicit what is being tested, and why a run might fail.

Move critical test code out of the event handler to clarify flow and
allow a query of the actual current workspace to use in assertions.

Works around an issue which caused this test to fail spurriously because
of pointer-related quirks in the i3 test suite which would sometimes
cause i3 to open on workspace 2 (However, the test is now agnostic to
the initial workspace or output).
Testcases: init pointer in a predictable position

Tests may disturb the pointer in their normal operation that may lead to
unexpected results in later tests using that display. Reset the pointer
before a test begins to (0, 0) to save test developers from related
"gotchas" and reduce multi-monitor test boilerplate.