lazy DCE load facade

[sashafirsov]

idea from: WICG/webcomponents#782
The DCE would be initialized only on actual tag use.

load-on attribute

accept values:

• declare or default value would load during declaration, disregarding of use
• use would create a facade which would load DCE when it is actually used. Of course only once.

facade pattern

would:

• register the tag, DCE source, fallback DOM as a payload.
• on tag appearance, render the fallback content
• initiate the DCE loading
• once loaded, replace the content

Advanced phases:

• payload would have multiple slots.
• default slot is shown immediately
• progress is shown when DCE is started to load but does not have a rendered content yet
• error - for whatever error reason
• load-error when DCE sources are not available

<custom-element scr="mod" tag="my-tag" load-on="use">
    <template>
        <slot> Wait my-tag to be loaded ... </slot>
        <slot name="error"> my-tag load error </slot>
    </template>
</custom-element>