Improve block unmount

[sergeche]

Currently, block content is unmounted in two steps:

1. Run dispose callback on block, if available
2. Detach block content: remove nodes and dispose other blocks

This causes several drawbacks:

• Inner blocks are disposed twice: first time in dispose callback, second time in emptyBlockContent runtime function.
• Leaked abstractions like $$animating key: element must not be detached until this property exists in element.

Block unmount must be refactored: always delegate block content unmount to generated function and emptyBlockContent must remove block from injector