[ALOY-1335] Use Alloy's deepExtend to handle conditional on-device style inheritance

Alloy selectors provide ways to overload style rules based on switches (formFactor, OS, conditional rules, etc.). Say the TSS is somehow:

...
".title-home": {
	bottom: 4,
},
".title-home[formFactor=tablet]": {
	bottom: 8
},
...

The generated code will look like:

Alloy.deepExtend(true, o, {
    bottom: 4
});
Alloy.isTablet && _.extend(o, {
    bottom: 8
});

If this works fine with most of the properties, this is particularly annoying with font or nested properties. For instance,

".title-home": {
	font: {
		fontFamily: "Delius-Regular",
		fontSize: 11
	}
},
".title-home[formFactor=tablet]": {
	font: {
		fontSize: 22
	}
},

will result in the font name being simply ignored on tablets, as _.extend() is not a deepExtend method. We're lucky, Alloy provides a deepExtend() implementation which we may want to use. I therefore propose to switch from _.extend() to Alloy's deepExtend implementation. This will allow much more efficient ways of integrating complex graphical interfaces, with style inheritances all across the app:

<Label class="fonticon icon-row-right icon-arrow-md icon-arrow-right white" />

All styles are merged deeply, and it becomes unnecessary to repeat over and over the font name. I _know_ that this change may break many tests, but I strongly believe that this behavior is the right way styles _should_ work, in the sake for development efficiency.