settings: replace CSettings and CGUISettings with new CSettings
settings: implement setting callbacks
fixes
add SettingLevel to CViewStateSettings
[skin.confluence] add sidebar in settings window for settings level and reset
update English strings.po with new strings
[win32] update VS project files
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>.
add <textureinvalid> in SettingsCategory.xml