Apply calculated height back after textblock fit

We were writing back a calculated width based on the flowing of the text
in the layouts, but not a height. That's corrected here now.
Simplifying Swt::TextBlock

Mostly this change revolves around getting our treatment of absolutes to
reach less deeply into the FittedTextLayouts, and sets the stage for
consolidating/extracting some of those parts.
Standardize on bounds instead of get_bounds
Method extraction and better naming
Adding TextBlockDimensions class
Use TextBlockDimensions from TextBlock
line_bounds instead of line_metrics
Use element_width instead of subtracting margins ourselves
describe instead of context
Merge branch 'master' into text_block_dimensions

Conflicts:
	lib/shoes/text_block.rb
Merge pull request #708 from shoes/text_block_dimensions

Extract TextBlockDimensions class. Changelog: improvement