CMediaSourceSettings: add PromptForSource() helper method
CMediaSourceSettings: add FindSourcesToSkip() helper
CMediaSourceSettings: add HandleSourceExistence() helper method
CVideoInfoScanner: remove unused member m_pathsToCount
CVideoInfoScanner: use std::set<std::string> instead of std::set<CStdString> for m_pathsToScan
video library: avoid slow library scanning when sources are not available

Avoid as many CDirectory::Exists() calls as possible on unavailable sources to
reduce possible timeouts.
music library: avoid slow library scanning when sources are not available

Avoid as many CDirectory::Exists() calls as possible on unavailable sources to
reduce possible timeouts.
videodb: add UpdateProgress() helper method
videodb: improve handling of unavailable sources in CleanDatabase()