remove instance of 'the the' in various places
when extension not found, offer to take user to extension downloads page

closes #328
core branch: remove some Help menu items
headless branch: remove moribund IntelliJ build
headless branch: remove sound extension

not really sure why I didn't do this before. I think it was because
the extension still compiled? but it isn't useful headless, so let's
axe it
headless branch: remove legal notices for libs we don't use in headless
headless branch: fix a small oops in shell.sh
headless branch: bring submodules.sh up to date
update models submodule
Merge remote-tracking branch 'origin/master' into core
Merge remote-tracking branch 'origin/core' into headless

Conflicts:
	dist/i18n/GUI_Strings_en.txt
	dist/i18n/GUI_Strings_es.txt
	dist/i18n/GUI_Strings_ru.txt
	dist/i18n/GUI_Strings_zh.txt
	src/main/org/nlogo/app/App.scala
	src/main/org/nlogo/app/HelpMenu.scala
minor cleanup
headless branch: remove some stray image files
Merge remote-tracking branch 'origin/5.0.x'

Conflicts:
	src/main/org/nlogo/agent/AgentSet.java
headless branch: no more headless subproject

since there's no GUI anymore
headless branch: oops
remove some 3D vestiges
remove some 3D vestiges
remove some 3D vestiges
whoops, make file name match class name