CUtil: add ValidatePort() method
XMLUtils: add std::string version of GetString()
WIP: guilib: add invalid input state to CGUIButtonControl

This adds HasInvalidInput(), SetInvalidInput(), GUI_MSG_INPUT_VALID and
GUI_MSG_INPUT_INVALID to CGUIButtonControl et. al. Furthermore the following
tags have been added for skins: <textureinvalid>, <alttextureinvalid> and
<invalidcolor>.
[confluence] add <textureinvalid> in SettingsCategory.xml
fix confluence
settings: add new settings system
settings: add (platform-specific) settings definitions
CApplication: integrate new settings system
CGUIWindowSettingsCategory: integrate new settings system
[skin.confluence] add a sidebar to SettingsCategory.xml
CWeatherManager: integrate settings callbacks
CRssManager: integrate settings callbacks
CPeripherals: integrate settings callbacks
libscrobbler: integrate settings callbacks
CWebServer: integrate settings callbacks [WIP]
CApplication: integrate settings callbacks [WIP]
fix CSettings
fix CGUIWindowSettingsCategory
fix settings
settings: add CApplicationSettings