The reader of CREDITS (GuiAbout.cpp) now ignores lines that start
with the number sign (#). This commit also introduces a commented
out header in CREDITS reminding any potential editor that all
changes will be overwritten by generate_contributions.py.
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.
