Use new display() values to remove some inset hardcoding.

Use new display() values to remove some inset hardcoding.

The enum is now made of flags that can be combined.

This introduces several new values for Inset::DisplayType:
BreakBefore, BreakAfter and Display=BreakBefore|BreakAfter. This
last value replaces AlignCenter.

Additionally the flags NoBoundary and CanBreakAfter are introduced for
future use.

Now a left aligned displayed inset will be defined as Display|LeftAlign.

A newline inset is characterized as BreakAfter.

This structure is used in breakRow to avoid explicit calls to
isNewline() or isEnvSeparator(). More improvements will be built on
top of this.

Additionally several redundant display() methods (which returned
Inline) have been removed.