This adds a warning icon to either the tab header or the panel stack
entry item if a widget on the panel/stack has invalid content.
Particularly helpful to get aware of such content on other tabs/panes
than the one currently selected.
In particular, the directory frontends/qt4 is renamed to frontends/qt.
Many configurations file have to be updated. All mentions of qt4 in
the source have been audited, and changed to qt if necessary.
The only part that has not been updated is the CMake build system.
