instead of sendSelectionNotifyEventToClient + setClientSelectionStage

Moving the client check to sendSelectionNotifyEventToClient enables us
to drop notifyConvertFailure.

and reset the lastClient variables when stage is set to None.

resolve lastClientClientPtr to process id and process name

Attributes GH PR #927: https://github.com/ArcticaProject/nx-libs/pull/927

In commit f48e2da3 I marked NX changes
by using #ifdef NXAGENT_SERVER. But I had missed that this define was
never set in mi. So let's change that now and make it work like
before.

Fixes ArcticaProject/nx-libs#926

Attributes GH PR #924: https://github.com/ArcticaProject/nx-libs/pull/924

Run newer compilers in Ubuntu Bionic 18.04.

Also update gcc from v9.x to v10.x, and clang from v8.x to 9.x

Fix some syntax changes in TravisCI configuration.
Signed-off-by: Mario Trangoni <mjtrangoni@gmail.com>

Attributes GH PR #896: https://github.com/ArcticaProject/nx-libs/pull/896

In rootless mode some properties are private (or internal or
adminitrative). They are only required for the windows on the real X
server side that represent nxagent's windows. Those properties should
never be cloned from there to the the nxagent windows so we filter
them.

Fixes ArcticaProject/nx-libs#920

more output, correct format specifiers

reformat, extend comments, simplify ifdef

helps to avoid valgrind warnings

reformat, whitespace, code simplification, FIXMEs

... to match it as closely as possible to the upstream version.

The autograb stuff had been forgotten.