Only expand selection on multiple clicks with the primary mouse button

Chrome + WebKit increment the click count value for the MouseEvent even
if the user switches buttons. E.g., a double click then right click
will report the last event as {button: 2, detail: 3}, causing the
selection to expand to the paragraph bounds.

Now, the click count is only reported as non-zero if the current event
is with the primary mouse button (left by default, or the right button
if the user has switched their primary mouse button around).
Add todo for existing flaw in mouse-selection click handling
Merge pull request #525 from peitschie/right-click-expand

Only expand selection on multiple clicks with the primary mouse button