Make capitalization more consistent

I'm not quite sure about these, for most you could make an argument to the contrary. For example, the sort box is technically a buttonmenu, and while the HIG technically also recommends sentence case for menu entries that are checkboxes or radio-buttons, that is probably an oversight and menu items should always be title case. Very willing to remove some or all of these changes.

@teams/vdg

There are a few places in Discover where capitalization doesn't seem quite consistent with the letter or spirit of the recommendations in the HIG. More specifically, the "Make default" button uses sentence case even though buttons generally fall under the rules recommending title case; for consistency the "Default Source" replacing the button should also be title case (and is arguably what the HIG would suggest). The sorting control uses title case; it's not quite clear what it should be but given that it functions like a combobox (even if it is technically not implemented as one) using sentence case should be in the spirit of the HIG.

This change makes the source button and label title case, and the fake combobox sentence case.

header	header