Dropdown fixes

This fixes the issue of Ctrl+6 (put dropdown) colliding with Ctrl+- (zoom out) on AZERTY keyboards mentioned in #581. It does this by binding the put dropdown action to Ctrl+Shift+M (for "put dropdown menu") instead.

The second commit inserts an extra separator in the put menu above the Dropdown option. This is mostly cosmetic, but IMHO also improves usability by making it clear to the user that this option is not part of the built-in vanilla GUI objects.

Note that the present changeset does not make the Ctrl+- (zoom out) shortcut actually work on AZERTY keyboards. However, it does resolve the issue of Ctrl+- rather unexpectedly inserting a dropdown menu into the current patch when using such a keyboard, instead of executing the expected zoom out action.

NOTE: Ctrl+- not working properly on AZERTY is due to the fact that on an AZERTY keyboard that key combination actually generates Ctrl+6 instead. Unfortunately, we can't just use localization to deal with this, since French keyboards may actually use various different layouts which put the - key in different locations. Thus right now it will still be necessary to manually edit the keybinding in pd_shortcuts.js (replacing key: "-" with key: "6") to make this shortcut work on AZERTY keyboards. If someone has an good idea on how to solve this in nw.js then I'm all ears. :-)