In Qt 5.15 many deprecation warnings are issued, presumably for
helping the port to Qt 6. They are really a load but can be
shuten up by using -DQT_NO_DEPRECATED_WARNINGS, except for the ones
about MidButton, AdjustToMinimumContentsLength, and DefaultLocaleLongDate.
This change should not affect compilation with Qt 4, as all changes are
backward compatible (only Qt versions prior 4.5 could be affected).
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.
