Improvements on export to dbms process. Now the process detects when a
object is being created, changed or dropped returning the correct
message to the user. Also, the sql buffer export was moved to a
separated method in order to be reused.
Added the step to export the diff to database on ModelDatabaseDiffForm

Minor adjustments on export to dbms procces on ModelExportHelper
Minor adjustments on modeldatabasediffform.ui

Minor bug fixes on ModelExportHelper, ModelsDiffHelper and
DatabaseImportHelper
Added the method PgSQLType::isEquivalentTo in order to check if a type
is equivalent to another. This method helps to avoids unnecessary
recreation of columns or type attributes on diff process.

Finished the first preview of model database diff process. Code
documentantion pending.
Merge branch 'export-improvements' into model-db-sync