Mistakes are visible but word replacement is disabled. Disable relevant
widgets depending on the situation.
Move updateView code to SpellcheckerWidget::Private.
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.
