Menu buttons should open menu on mouse-down

[martinmcclure]

SpkMenuButtonPresenters currently act like Pharo's world menu -- the menu opens on mouse-up, then you can click on the menu item you want, whose action is executed on that mouse-up. Thus, it always requires two clicks.
Pharo's window menu (at the far right of each window's title bar) has the same problem.

Sparkle's menu buttons should act like Pharo's menu bar menus -- the menu opens on mouse-down. You can then either release the mouse button and select the menu item with a second click, or drag the mouse to the desired item and release the mouse button. The second option lets you select menu items in a single mouse operation, rather than having to click twice.