Introduce Style element

[lippserd]

This PR introduces a new Style element to provide strict CSP compliant dynamic CSS when necessary.

Note that it is still a work in progress, so it is not in the right namespace, is not documented, API is subject to change, and lacks unit tests.

Example of creating style with some rulesets:

$style = new Style();
$style->add('.size', [
    'height' => '1em',
    'width'  => '1em'
]);
$style->add('.border', [
    'border' => '1px solid black'
]);

Output:

<style>
.size {
  height: 1em;
  width: 1em;
}
.border {
  border: 1px solid black;
}
</style>

Now we wrap the rules in a selector:

$style->setSelector('.parent');

Output:

<style>
.parent .size {
  height: 1em;
  width: 1em;
}
.parent .border {
  border: 1px solid black;
}
</style>

We can also use LESS variables and nested rulesets:

$style->setProperty('@black', 'black');
$style->addRuleset(
    LessRuleset::create('.widget', ['color' => '@black'])
        ->add('&.detail', ['font-size' => 'bigger'])
);

Output:

<style>
.parent .size {
  height: 1em;
  width: 1em;
}
.parent .border {
  border: 1px solid black;
}
.parent .widget {
  color: var(--black, #000000);
}
.parent .widget.detail {
  font-size: bigger;
}
</style>

Style can be automatically wrapped in a module scope:

$style->setModule(Icinga::app()->getRequest()->getModuleName());

Output:

<style>
.icinga-module.module-icingadb .parent .size {
  height: 1em;
  width: 1em;
}
.icinga-module.module-icingadb .parent .border {
  border: 1px solid black;
}
.icinga-module.module-icingadb .parent .widget {
  color: var(--black, #000000);
}
.icinga-module.module-icingadb .parent .widget.detail {
  font-size: bigger;
}
</style>

And for conformance with strict CSP, a nonce can be set:

$style->setNonce(Csp::getStyleNonce());

Output:

<style nonce="9MEcubtLrEm7isUUbl/l0A==">
...
</style>

[lippserd]

@nilmerg I'd like to discuss API and namespace.

[nilmerg]

Thinking about using lessc directly, as suggested by @yhabteab. Inline style is mainly used for layout purposes and for this, theming and mode support isn't relevant. If we don't support that this way, devs are encouraged to define proper styles.

[nilmerg]

Since it doesn't depend on Icinga Web anymore, the namespace should be fine as it is now.