Extend xml:space to support "remove" for TextBlock and RichTextBlock Run Collections
This is an extremely easy and non-breaking way to solve the long-running problem of undesired spaced between <Run> elements in a <Run> collection. Not having inner text between the <Run> elements in the XAML source doesn't really work because generally we want the XAML editor to support reformatting by nicely breaking <Run> elements onto separate lines. Extending xml:space to support "remove" means existing code will not be broken, but developers can add the tag to instruct the parser to discard the entire inner text if it is all whitespace. If MS is worried about a conflict with possible future XML spec changes, then introduce an MS-specific tag that performs the same function.