Improve loading of files from https://www.eclipse.org/papyrus

umbrello/uml.cpp
- In function slotFileOpen call to QFileDialog::getOpenFileUrl argument
  `filter' add pattern "*.uml" for Eclipse PapyrusUML files.

umbrello/uml1model/association.cpp
- In function load1:
  - Use function Model_Utils::getXmiId to abstract away the difference
    in XMI ID attribute (UML1 "xmi.id" vs. UML2 "xmi:id").
  - Support loading of association content without an envelope element
    such as <Association.connection>, <Association.end> etc.
  - Prepare for supporting PapyrusUML element <ownedEnd>.
- In function saveToXMI1, if m_AssocType is a Generalization then do not
  produce unused/empty XML attribute "discriminator".

umbrello/version.h
- Increase XMI_FILE_VERSION to "1.7.4" due to changed <UML:Association>.

CCBUG: 56184