Change Shift+Space to Ctrl+Space for inserting syllable breaks

[1ec5]

We should replace the Shift+Space (⇧Space) shortcut for inserting syllable breaks with Ctrl+Space (⌃Space).

Firefox for the Mac has removed the nonstandard ⌃Space shortcut for opening the context menu (apparently in favor of fn⌃I or ⌃5 on the numpad with Mouse Keys enabled). Ctrl+Space is far more common than Shift+Space as a keyboard shortcut for switching between input methods or toggling input method options on both OS X and Windows. Ctrl+Space is also far less likely to be pressed by accident before typing a capitalized word – this still gets me once in awhile after all these months – and it requires fewer hacks.