Fix placement of webengine process

In developer (non-prefix) builds, the binary goes to $$MODULE_BASE_OUTDIR/libexec,
which is practically qtbase/libexec. In prefix builds it'll be linked to $module_toplevel/libexec
and installed into $prefix/libexec.

Change-Id: Ia42519f3113976e707fbda9e09dbf7ef6e235924
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>
Fix installation of most examples

* Make sure the example source code is installed
* Make sure the installation path is consistent with the path within the module, to ensure
  that the sources end up next to the compiled binary.
* Skip the widgets nano browser from the installation, as it's a developer tool.

Change-Id: I35b421e12f8e6ddd3f2045e318b21543ff929cd8
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>