Improve look of FormLayout section headers

The current styling looks a bit unpolished with its small left-aligned label. Its small size and alignment does not feel like a section header. This commit improves that.

Here's a before-and-after with plasma/kinfocenter!57 (merged) (which does the right thing by actually using section headers, rather than rolling its own to work around the current aesthetic issues, as most FormLayout users currently do):

Before	After

cc @teams/vdg