settings: add CSetting::SetCallback() instead of using friendship with CSettingsManager
settings: add cosmetic comment to CSetting
move ISliderCallback into its own header file
guilib: don't abuse and redeclare SPIN_CONTROL_TYPE_FOO
WIP: settings: add slider control
WIP: settings: add CGUIControlSliderSettings
WIP: json-rpc: add new settings slider control
settings: add CGUIDialogSettingsBase
settings: add CGUIDialogSettingsManagerBase
CSettings: expose CSettingsManager instance
settings: update CGUIWindowSettingsCategory to derive from CGUIDialogSettingsManagerBase
[skin.confluence] add slider control to SettingsCategory.xml