[FEATURE] Add options to set bold for legend layer, groups

- Based of off work from Stefan Ziegler (pull request #331)
- Options inherited by embedded groups (which are still always italic)
- [API] Add originalName() to QgsMapLayer to access original (pre-capitalization name)
- [API] Add setupFont() method to legend group items
- [API] Add updateLegendItemStyles() to QgsLegend to allow new option (and capitalization) changes to become active without app restart