Blockwise yank and paste operations initial implementation.

- Changed the way model offset is calculated based on visual offset to
  fix issues when the line is not visible.
- Added custom system clipboard content so text blocks can be put there.
Blockwise paste: don't right-pad at the end of line.
Blockwise paste: fixed paste at EOL.

Because line information is not updated after insertion the
check has to be less than.
Fiexed type in Tex*c*tBlockContentBuilderRegister
Merge pull request #246 from igrekster/blockwise-yank-paste

Blockwise yank and paste operations initial implementation.