Parse template initial attribute values as properties, not attributes

[pleku]

Follow up on #8957, given template with contents

<vaadin-text-field id="name" placeholder="John Doe" label="Name" autofocus max-lenght="30"></vaadin-text-field>

the initial attribute for the element should be mapped as properties*. Thus

@Id("name") TextField nameField;

nameField.getPlaceHolder(); // returns "John Doe"
nameField.getLabel(); // returns "Name"
nameField.isAutoFocus(); // returns true
nameField.getMaxLenght(); // 30

As can be seen for instance with https://vaadin.com/components/vaadin-text-field/html-api/elements/Vaadin.TextFieldElement the web component API has properties, but not attributes. So this needs to be the default.

*The special cases that are not properties are at least:

• id (handled by Read attribute value from template #8957)
• class (handled by Read attribute value from template #8957)
• style (handled by Read attribute value from template #8957)
• theme (handled by Read attribute value from template #8957)
• disabled (handled by Parse initial disabled attribute value from templates #8973)
• maybe something else, can be changed later on when discovered

[denis-anisimov]

This ticket may have dramatic impact on the templates behavior .

With the current implementation (exactly as described) the following happens:

• template has an element with hidden : <div id='injected' hidden></div>.
• hidden is read as a property (it's not in the list , so it's not read as an attribute EVEN though this is an attribute by HTML spec).
• Flow treats empty values for attribute so that if it has "" value then its value is true ( and as a result it will be rendered as hidden) otherwise it will be removed. And hidden attribute will work correctly. But hidden property doesn't work since property value is set to "" and send to the client side where it's set to the same value. BUT this is not a property. This is an attribute. And HTML removes the attribute from the DOM .

What we should do : treat hidden as an attribute and not a property.
And it's obvious that hidden is just a special case. If some other attribute with boolean semantic is treated incorrectly as a property then we are introducing bugs.

[denis-anisimov]

Should this be used as a reference for global attributes? :
https://www.w3schools.com/tags/ref_standardattributes.asp

I'm going to add hidden as an attribute otherwise there is an IT test which just fails because of that.