feat: Add function numberOfInstances()

[ouuan]

This function returns the number of running instances, by adding a new variable in InstancesInfo.

This closes #89.

[ouuan]

I met a problem: The destructor is not called after ::exit() is called, which is called in QCommandLineParser when the option contains --help, --version or invalid options, thus the count is not updated. I found that the destructors of variables declared with static will be called after ::exit(), but that's not recommended for QApplication. Do you have any solutions?

[ouuan]

What's more, it would fail if one instance crashed.

[ouuan]

I have a solution which can deal with QCommandLineParser, but not crash:

Use qAddPostRoutine to add a function which reduces numberOfInstances.

[ouuan]

What about use LocalServer to send a message and wait for replies from other instances in 100ms?

[ouuan]

I came up with a trick: Since my actual need is to know whether the current instance is the only running instance, call detach() and check if the shared memory is destroyed should work.

[ouuan]

I don't know which solution is the best, or if there are better solutions. However, this PR may be useless. Close this if you like, or tell me how to improve it so that it works correctly after a crash.

[itay-grudev]

Yhea. An instance crashing is a serious problem I had to deal with. here is not much you can do about it. Also the Qt Q[Core]Application constructors and places where they just call ::exit() internally. You may be correct that number of instances maybe really hard to achieve with exception to pinging other instances to check for activity, which solution I don't really like because it consumes resources.

I'll think about it for a bit and if I can't come up with an alternative I will close it.

[ouuan]

Close this in favor of #91