Add RTL support

[ronnetzer]

This PR includes several fixes for RTL:

• [x] Overall table borders and paddings
• [x] Meta rows horizontal scroll calculation
• [x] Column resize calculation, handler styles, and position
• [x] Context menu position

Added an 'Rtl Demo' under demos dropdown, as you can see it's still not perfect: when you scroll down the meta rows flips (I think its the sticky rows, I've tested it with the sellers-example and it was ok)

Also, added an unrelated fix for .pbl-ngrid-row-visually-hidden: when using pbl-ngrid-spacing mixin, the mixin adds a rule that is more specific than .pbl-ngrid-row-visually-hidden which overrides its 0 height/min-height

[shlomiassaf]

@ronnetzer Amazing work

I'll add a dedication documentation page later, instead of the global demo so it will have some documentation!

Thanks!

[shlomiassaf]

As for the sticky issue, the whole sticky rows thing does not fit virtual scrolling anyways, it's not built for that and the CDK will have to undergo a major refactor to support it.

This is why I created the fixed meta-rows model.

I can live with it for now.

BTW, I only see it when I scroll, my guess is that some CSS class that appear on the grid while scrolling does that Or it could be some logic that freezes the top fixed metaheader row while scrolling, pushing it to the end or something.

That's the thing about grid's, so many features need to play along together