We tend to have insets which buffer() member is invalid. To help
debugging, this commit paints their background in red when devel-mode
is on.
To this end, a new method develMode() is added to the Painter class.
With this commit, it is easy to see that macro template do not have a
proper buffer set!
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.
