[bind] Implement bind lookups

[jkb0o]

For now only simple binds are supported:

from!(player, Player:name) >> to!(name, Label:value)

The problem is if I create something like Profile widget which displays player avatar/name/health based on provided Player entity, it would be difficult to handle binds.

The idea is to respect the changeable page.player entity in the single bind like this:

from!(page, Page:player @ Player.name) >> to!(name, Label:value)
from!(page, Page:player @ Health:current) >> to!(bar, Range:value)
from!(page, Page:player @ Health:max) >> to!(bar, Range:maximum)

This is how @ lookup selector is supposed to work:

• assume that Page.player of the Entity type
• bind Player.name at Page.entity to name, Label.value (and others)
• if Page.player changes - clear existed binds from Page.player and rebind them to new Page.player source.

[jkb0o]

This will be done right in polako (#77)