Add CSS pseudo-class 'nonempty' for workspaces containing at least one window

Still needs some work, but it's a start. In particular:
* only applies to grid mode, not single-row mode
* only useful after adding the 'nonempty' pseudo-class to the Cinnamon theme CSS

Here's the relevant section from cinnamon.css:
```css
#workspaceSwitcher {
	spacing: 0px;
	padding: 3px;
	
}
.workspace-button {
	width: 20px;
	height: 10px;
	color: #cccccc;
    background: #888;
	border-left: 1px;
	border-top: 1px;
	border-color: #555;
	padding: 2px;
}
.workspace-button:outlined {
	padding: 1px;
	background: #dd7700;
	color: #555;
}
.workspace-button:nonempty {
	padding: 1px;
	background: #bbb;
	color: #555;
}
.workspace-button:outlined:nonempty {
	padding: 1px;
	background: #ffb048;
	color: #555;
}
```