refactor dialog: observe id instead of watching

Id attribute should only be modified outside directive. So, we should
observe the attribute instead of watching the attribute.
refactor dialog: remove useless destroy method
feat dialog: add scrollable behavior

When dialog is higher than window height, actions are fixed to window
bottom and content is scrollable.
Moreover, dialog style is more flexible with customizable header.

BROKEN:
dialog__title class is removed in favor of dialog__header.
dialog__header should prepend dialog instead of being inside
dialog__content.
Merge pull request #78 from lumapps/feat/dialog

feat dialog: add scrollable behavior