TextAlign also for the table header of data grid

[NoOneKnows92]

Added the possibility to also align the table header of the data grid if wanted, which can be seen as a fix to #1000 maybe.

[stsrki]

I'm getting errors AlignHeader does not exist. Looking at the code I think it's not needed and probably you left it by accident?

[NoOneKnows92]

I added this property to fit the current documentation where it says that the property TextAlignment only aligns the display cell and if someone wants to align the header alongside the body it needs to be set over the property AlignHeader. This would be my understanding of the current documentation and how it can be enhanced without leaving someone behind.

[stsrki]

Instead of using AlignHeader I have added new HeaderTextAlignment property. This way we have more freedom to align the header independently of the cell. Also, I created a new TableHeaderCellTextAlignment method so that it doesn't use the same code as in the cell.

All in all, good job and thank you :)