QFontMetrics::leftBearing & rightBearing, add handling for 32bit unicode chars

QFontMetrics functions leftBearing & rightBearing both take a QChar as an argument which limits the char to 16 bit. Currently there seems to be no way to get the bearings for 32bit unicode chars which can't be expressed in 16 bits (such as certain emojis).

Suggestion: add functions to get leftBearing and rightBearing for all 32bit unicode chars.