StashBuildTrigger: Rename StashBuildTriggerDescriptor to DescriptorImpl

[proski]

*  StashBuildTrigger: Rename StashBuildTriggerDescriptor to DescriptorImpl
   
   DescriptorImpl is the standard name for a descriptor defined as an inner
   class. StashBuildTriggerDescriptor is a long name that duplicates the
   name of its containing class, StashBuildTrigger.

Actually, StashPostBuildComment.java uses DescriptorImpl, so the plugin code is inconsistent. Let's make it consistent.

The descriptor name is written to the XML file that holds the global configuration of the plugin. That's where the option to enable pipelines would go. That's why I want this PR to be merged before #69 so that we don't deal with backward compatibility later.

[jakub-bochenski]

The descriptor name is written to the XML file

Don't we need a data migration then?

[proski]

We don't have any data to migrate because we don't have any global settings yet. I actually tried upgrading from the master branch to this PR, there are no messages about unreadable data.

The goal is to have global setting use the shorter and idiomatic name from the beginning. That way we avoid data migration in the future.

[jakub-bochenski]

We don't have any data to migrate because we don't have any global settings yet. I actually tried upgrading from the master branch to this PR, there are no messages about unreadable data.

OK, I was worried the empty descriptor would still generate warnings