Load the whole analysis when a BEAST XML file is dropped on BEAUti (#141)

[alexeid]

Fixes #141.

Dropping an XML file on BEAUti imported its alignment only: every dropped file went
to the alignment importers, and XMLImporter pulls just the alignment out of a BEAST
XML, so the model and MCMC settings were discarded.

What changed

• BeautiDoc.isBeautiAnalysisFile(File) sniffs a file for the beautitemplate attribute
  on its top level element -- the same marker extractSequences uses to pick a template.
  That is what tells a BEAUti generated analysis apart from an alignment file.
• Dropped files that pass that test are loaded exactly the way File/Load loads them
  (newAnalysis + loadXML), so model and MCMC settings come back. Anything else still
  goes to the alignment importers, so dropping NEXUS/FASTA/BEAST1 files is unchanged.
• Drops are accepted anywhere on the BEAUti window, not only on the partitions table.
  Dropping a non-analysis file outside the partitions table now says where alignments go
  instead of silently doing nothing.
• The File/Load body moved into BeautiTabPane.loadFile(File) so the menu and drag and drop
  share one code path (save buttons enabled, window title, error dialog).
• If several files are dropped, only the first analysis is loaded (loading one replaces
  everything else) and BEAUti says so. If an analysis is already loaded, it asks before
  replacing it -- easy to drop a file by accident, less so to pick File/Load by accident.
• Loading is posted to the FX thread with Platform.runLater, because it rebuilds every
  panel including the one the drop came from.

Testing

• DragAndDropXMLTest sets up an analysis (HKY, non-default chain length), saves it,
  starts a new analysis and then drops the saved file: the alignment, the site model and
  the chain length all come back.
• BeautiAnalysisFileTest covers the file sniffing: BEAUti XML, BEAST XML without
  beautitemplate, NEXUS, missing file.
• Full beast-fx suite passes.
• Native drag and drop itself cannot be driven from TestFX, so the actual drag still wants
  a manual check: drop a saved analysis on various tabs, drop a NEXUS file on the partitions
  table, drop a non-BEAUti XML.