Followup to b383c69. Keep labeling dialog from calling init() twice from vector props dialog
Check only what's necessary to see if a layer will be labeled

- Don't load all settings from QgsPalLayerSettings::readFromLayer
Add QgsFontUtils class to handle system font queries and style changes
Update label gui and rendering with QgsFontUtils functions
Add user notification when defined labeling font is not found on system, then substituted

- [API] add signal to QgsVectorLayer that is emitted when its labeling font is not found
- Message bar notification on first rendering of layer offers link to open layer's labeling dialog
- Add 'font not found' notification to labeling dialog
- Labeling dialog defaults to 'Text style' section when font is not found, to show notice
- Substituted font is not saved with project, unless user applies changes.
- Change labeling gui font selector dialog to font family combobox (since only family is being chosen)