AppPropertyGrid now allows text highlight in value cells

This feature was actually attempted before but was not working because
it was trying to set a style attr which was not able to affect the child
tags which had unselectable classes. The new method uses a proper css
class (see 'yes-text-select' - css class name chosen based on existing
'no-text-select' class)

See also comments in the css file covering known caveats/limitations...