Performance improvements and better compatibility for IntelliJ 15

• Performance improvement: render preview only when visible
• Don't show AsciiDoc preview actions in other Editors
• additional compatibility fixes for IntelliJ 15.x

make compatible with IntellJ 15.x again

Version 0.14 worked with IntelliJ 2016, but didn't work with IntellJ 15.x. This update uses APIs available in both versions.

JavaFX preview, Update asciidoctorj-diagram

• New JavaFX preview and real split view
• Update to asciidoctorj 1.5.4.1 and asciidoctorj-diagram 1.5.0

JavaFX preview, Update asciidoctorj-diagram (preview)

• New JavaFX preview and real split view
• Update to asciidoctorj 1.5.4.1 and asciidoctorj-diagram 1.5.0