Avoid deprecation warnings

Avoid deprecation warnings

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).