Add jmri.display load&store tests

[bobjacobsen]

See Issue #7528 and PR #7537

Although we had XML files in the test tree that contained Portal elements, they were only being verified. There was no load-store-compare test.

This PR provides that by adding a LoadAndStore implementation to jmri.jmrit.display, along with load/ and loadref/ directories for two test files.

This was branched from v4.17.3, to be earlier than the recent portal changes: It's meant to catch anything those broke. By itself it passes all tests, but it might highlight problems in a merge. We'll see what CI does.

[bobjacobsen]

The changes in Portal load/store since 4.17.4 are being tagged as errors by this PR:

[ERROR] Failures: 
1261[ERROR] jmri.jmrit.display.LoadAndStoreTest.loadLoadStoreFileCheck[java/test/jmri/jmrit/display/load/PanelEditorTest1.xml (pass=true)](jmri.jmrit.display.LoadAndStoreTest)
1262[ERROR]   Run 1: LoadAndStoreTest>LoadAndStoreTestBase.loadLoadStoreFileCheck:293->LoadAndStoreTestBase.checkFile:212 expected:<  <signalmast[s class="jmri.managers.configurexml.DefaultSignalMastManagerXml" /]>> but was:<  <signalmast[logics class="jmri.managers.configurexml.DefaultSignalMastLogicManagerXml"]>>
1263[ERROR]   Run 2: LoadAndStoreTest>LoadAndStoreTestBase.tearDown:312 Unexpected ERROR or higher messages emitted:          to file2:"/home/travis/build/JMRI/JMRI/temp/temp/PanelEditorTest1.xml"
1264[INFO] 
1265[ERROR] jmri.jmrit.display.LoadAndStoreTest.loadLoadStoreFileCheck[java/test/jmri/jmrit/display/load/PortalCheck.xml (pass=true)](jmri.jmrit.display.LoadAndStoreTest)
1266[ERROR]   Run 1: LoadAndStoreTest>LoadAndStoreTestBase.loadLoadStoreFileCheck:293->LoadAndStoreTestBase.checkFile:212 expected:<  <[signalgroups class="jmri.managers.configurexml.DefaultSignalGroupManagerXml" /]>> but was:<  <[oblocks class="jmri.jmrit.logix.configurexml.OBlockManagerXml"]>>
1267[ERROR]   Run 2: LoadAndStoreTest>LoadAndStoreTestBase.tearDown:312 Unexpected ERROR or higher messages emitted:          to file2:"/home/travis/build/JMRI/JMRI/temp/temp/PortalCheck.xml"

[bobjacobsen]

These changes were merged as part of #7552