Sandcastle tweaks

[mramato]

Added the below two functions to Sandcastle and made the relevant demos include them.

1. Sandcastle.addDefaultToolbarButton executes and highlights the toolbar immediately, eliminating the need for breaking out an extra function to call (muddying the code in the process).
2. When defined by a demo, Sandcastle.reset is called whenever a button or menu selection is main. This allows for the bookkeeping/cleanup code to be cleanly abstracted away from the example-specific code that the user is interested in.

[emackey]

This needs a merge from master now, and edits to the GeoJSON example.

[mramato]

Yep, doing it now. I also realized I need a change to the default/reset logic due to a problem exposed by the updated GeoJSON example.

[mramato]

Okay, ready.

[emackey]

Got an idea, doesn't have to be for this PR, but let me know what you think:

var viewer = new Cesium.Viewer('cesiumContainer');
var scene = viewer.scene;
var miscMenu = Sandcastle.addToolbarMenu('Misc Materials');

miscMenu.addDefaultOption('Rim Lighting', function() {
    // Cesium example code here
});

miscMenu.addOption('Water', function() {
    // Cesium example code here
});

Basically, by declaring menu(s) at the top, and then adding options individually to the menus, we can make the options get added similar to how buttons get added. This would avoid them needing .declare and .highlight, and the sample code for an option would be in the same place in the source where it gets added. It would remove the "lists of options" filling up space at the bottom of the example.

What do you think?

[mramato]

Great idea. Are you going to take a stab at it? If not I can probably do it over the weekend (in a different PR).

[emackey]

Not all menus have a header at index 0, for example the Billboards menu.

[mramato]

Okay, I improved the check so it doesn't matter now. This is ready.

[emackey]

@mramato I didn't get any time to work on the menu feature today. Let me know if you attack it this weekend, otherwise I'll try to carve out time next week.