Modular hotkeys

This pull request introduces logic to prevent BigBlueButton from accepting hotkeys for modules which aren't loaded, and from displaying those hotkeys in the Shortcut Help Window. 

For example, if a particular BBB server has the Presentation module removed from config.xml, then the global hotkey for "Focus to Presentation Window" won't even attempt to do anything, won't appear in the General Shortcuts section of the Help window, and the entire Presentation category will not appear in the Help window drop-down list.