fix missing abstract keyword on AbstractPsiParsingAndGenerateSourceTest class
add note about integration of DAMapping extensions
add ShortNameCache + hardcode support for test project generated classes

hardcoded support is kind of required because there must be only one instance of PsiClass representing a generated class
format pom.xml files content
statically caching generated PsiClass is actually not necessary

still, generating the same PsiClass multiple times if not efficient, need to be cached/indexed
add Indexes of *Mapper classes by name and qualifiedName

using index to drastically improve perf of PsiShortNamesCache implementation
using index to improve ElementFinder implementation by qualifiedName