Change-Id: I8b73d583be1ee7183f4074bce49d5390e38631a2

Remove duplicated classes ProjectExplorer::FileWatcher
and QmlProjectManager::FileSystemWatcher, create
Utils::FileSystemWatcher from them, merging the functionality.

Also use in HelpManager/Maemo, reducing the number
of QFileSystemWatcher instances (and thus, shutdown time).