Add style to secondary action buttons

[guimspace]

This PR adds styling to buttons with class "secondary" to whenever they are secondary actions. For example, in a form with buttons Save, Reset, and Cancel, Save is the primary action whereas Reset and Cancel are secondary.

• I didn't add the style to input[type="submit"] because (usually) they are the primary action.
• I first tried to make a secondary action with a gray background, but it didn't work out well and got mixed up with the disabled buttons. So I simply made the background transparent.
• The disabled mode of the secondary buttons don't look so great, so perhaps you can ignore the second commit and use the existing style to both primary and secondary actions.

I tested the changes with theses buttons from the Demo page:

<p>
	<input type="submit" value="<input type=submit>">
	<input class="secondary" type="submit" value="<input type=submit>">
</p>
<p>
	<input type="button" value="<input type=button>">
	<input class="secondary" type="button" value="<input type=button>">
</p>
<p>
	<input type="reset" value="<input type=reset>">
	<input class="secondary" type="reset" value="<input type=reset>">
</p>
<p>
	<input type="submit" value="<input disabled>" disabled>
	<input class="secondary" type="submit" value="<input disabled>" disabled>
</p>
<p>
	<button type="submit">&lt;button type=submit&gt;</button>
	<button class="secondary" type="submit">&lt;button type=submit&gt;</button>
</p>
<p>
	<button type="button">&lt;button type=button&gt;</button>
	<button class="secondary" type="button">&lt;button type=button&gt;</button>
</p>
<p>
	<button type="reset">&lt;button type=reset&gt;</button>
	<button class="secondary" type="reset">&lt;button type=reset&gt;</button>
</p>
<p>
	<button type="button" disabled>&lt;button disabled&gt;</button>
	<button class="secondary" type="button" disabled>&lt;button disabled&gt;</button>
</p>
<p>
	<a href="#forms__action" class="button">&lt;a class=button&gt;</a>
	<a href="#forms__action" class="button secondary">&lt;a class=button&gt;</a>
</p>
<p>
	<a href="#forms__action" class="button" aria-disabled="true">&lt;a class=button aria-disabled=true&gt;</a>
	<a href="#forms__action" class="button secondary" aria-disabled="true">&lt;a class=button aria-disabled=true&gt;</a>
</p>

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[kevquirk]

This feels like something that's firmly in the custom CSS space, and probably shouldn't be included in the upstream project. I'm very reluctant to add additional classes. @lkhrs what do you think?

[lkhrs]

I agree with Kev, we want to minimize additional classes.

[guimspace]

Hey, thank you for the feedback, guys! This PR really felt like pushing the edge when I submitted it, but needed to share the idea anyways.