Use (NSFont*)convertWeight:ofFont: to get bold font

This changes the behavior for fonts with many weight variants.
Currently, if a font has the variants Light, Medium and Bold and user
normally uses Light, the Bold font will be jarringly bold. With this
change, Medium will be selected instead which fits better visually.
Merge pull request #189 from calmh/use-convert-weight

Use -convertWeight:ofFont: to get the next bolder version of a font instead of getting the "bold" version, which might be too bold (i.e., when using Mono Extra Light, then the "regular" weight should be used for bold text, as it's the next step up in boldness).