Bugfixes, technical improvements and documentation

Task #1: Restructure repository to work with Freeplane Gradle plugin
Task #4: Update add-on definition mind map to latest devtools version
Bug #3: Accelerator modifier not cross-platform
Bug #2: Execution of scripts freezes when opening node editor
Task #6: Remove obsolete line "// @CacheScriptContent(true)" from scripts
Bug #7: Issue can refer to a node that is not an issue, position or argument
Task #5: Document add-on in README.md

Initial release by Lynn C. Reese

This is the initial release by the original developer Lynn C. Reese. It was developed for Freeplane 1.2.15 and higher. It does not work properly with recent releases of Freeplane.