Fix compile warnings, introduce TabList::current_is_valid 

src/Widget.cpp: In constructor ‘TabList::TabList(ScrollType)’:
src/Widget.cpp:53:39: warning: declaration of ‘scrolltype’ shadows a member of 'this' [-Wshadow]
src/Widget.cpp: In member function ‘Widget* TabList::getNext(bool)’:
src/Widget.cpp:92:44: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
src/Widget.cpp:100:29: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
src/Widget.cpp: In member function ‘Widget* TabList::getPrev(bool)’:
src/Widget.cpp:112:44: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
src/Widget.cpp: In member function ‘void TabList::activate()’:
src/Widget.cpp:130:44: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
src/Widget.cpp: In member function ‘void TabList::defocus()’:
src/Widget.cpp:137:44: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]