Merge pull request #257 from jhgg/master

Fix bitmath issue with borderless flag in textgrid:sethasborder()
* implement textgrid:sethastitlebar() + other things

* Make it so that text grid can always become a key/main window (if it doesn’t have a title bar)
* textgrid:center() method too.
Merge pull request #258 from jhgg/textgrid-sethastitlebar

implement textgrid:sethastitlebar() + other things
Properly index fnutils.{sequence,partial}