it's confusing for the users if the examples' project files contain code
to install their own sources. also, this constitutes an enormous code
duplication, and lots of mistakes. consequently, automate it.

more or less as a side effect, this also removes the entirely meaningless
target installs in subdirs projects.

Task-number: QTBUG-28184
Change-Id: I9fc1367a06db9e2c46aeb67d68729a4f67163ef9
Reviewed-by: hjk <qthjk@ovi.com>