Before, we would continually set the selection to the empty string,
to maintain ownership. This causes problems with clipboard managers
GPaste and CopyQ. Further, in theory this should have better
performance since checking ownership should be faster than setting.
A disadvantage of this change is that the timestamp of the selection
will be old at this point in the code. However, when the selection
is requested, we put the selection contents at that point so the
selection timestamp should be accurate then. Further, the selection
contents are not accurate at this point in the code (they are just
the empty string), so it is not clear whether having an accurate
timestamp is beneficial.
Fixes#11715.
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.
