Add PopupComponent

[WestHuus]

The PopupComponent should be used for to positioning content like tooltip or popover automatically in the right place near a trigger.

A popup mainly consists of a trigger (the Element(s)) which calls the content .
It can cen configured by

• offset the space (in px) between trigger and content
• flipping if no space on chosen available it will be find a right placement automatically
• placement of the content around the trigger

The trigger provides two handler which can be used, the first is important to open/toggle the content the second close it.
content provides one handler which can be used to close it.

Example:

popup {
    offset(10.0)
    flipping(false)
    placement { topStart }
    trigger { toggle, close ->
        span {
            +"hover me"
            mouseenters.map { it.currentTarget } handledBy toggle
            mouseleaves.map { } handledBy close
        }
    }
    content { close ->
        div {
            +"my content"
            clicks.map{ } handledBy close
        }
    }
}

resolves #447

this PR needs #486 and #481 before it can be merged