Add a window width option for use when toggling sidebar

It would be very handy if there was a simple boolean option that would cause the editor to resize itself when the sidebar is toggled.

For instance, say that the editor window does not have the sidebar visible and it's currently 800px wide. Let's also assume that the sidebar was previously set to 200px.

When the new feature that I'm proposing is enabled and the sidebar hotkey is pressed, the editor window would get the width of the sidebar and resize itself to be that much wider on the side where the sidebar appears. So the total window width would be 1000px. When the sidebar is closed, the window would decrease back down to 800px.

This behavior is roughly similar to how the old Textmate 1.x file drawer functioned - the drawer could open without affecting the placement of the rest of the window. This would be very very handy for people that don't use their editor in fullscreen mode.