[JBIDE-15598] added support for verifying if the appropriate environment variable related links are present
[JBIDE-15598] added tests for IEnvironmentVariable#destroy

* renamed hasListLink -> canListEnvironmentVariables
* rename hasUpdateLink -> canUpdateEnvironmentVariables
* renamed IApplication#getEnvironmentVariableByName to
#getEnvironmentVariable (and analog methods)
* switched environment variables from list to map
* removed link-testing methods from IEnvironmentVariables
* Added removal tests