Content Model fidelity improvement 3: Block format

[JiuqingSong]

I'll make a series changes to improve content model fidelity.

In this change I reorganize how block format is handled. Now we have 3 categories of block formats:

1. Inheritable formats: declare once, affect all its children nodes, until it is overwritten. These styles will be kept in context and apply to all single blocks under the element where it is declared.
   • whiteSpace
   • direction
   • lineHeight
2. Onetime formats: when declared, it will only impact the element where it is declared. In its child nodes these styles will not be preserved. We will use ContentModelFormatContainer to represent block groups with these formats.
   • margin-top
   • margin-bottom
   • padding-top
   • padding-bottom
   • border-*
   • backgroundColor
3. Accumulateable formats: if declared in multiple layers, its value will be accumulated, until a FormatContainer is hit.
   • margin-left
   • margin-right
   • padding-left
   • padding-right

With this design, now I split original knownElementProcessor into several pieces:

• blockDecoratorProcessor: Handle P, H1-H6 that is actually a decorator of a paragraph, then call knownElementProcessor to process its content. Now we will put block decorator into context so that other processor can also leverage it
• linkProcessor and codeProcessor: Handle A and CODE that is actually a decorator of a segment, then call knownElementProcessor to process its content
• formatContainerProcessor: Handle container-like element, includes PRE, BLOCKQUOTE (with top or bottom margin) and ohter block elements that has top/bottom margin/padding or background color.
• knownElementProcessor: Handle normal paragraph and segment, then call childProcessor to handle content.

In order to make test case easier to write, I added an option to skip caching element for table when create Content Model.