Examples: Better check for non-writable location

Checking the .pro file is actually not too relevant, so extend the check
to the .pro files directory, and more importantly, to the directory
above, which will be used as the default location for shadow build
directories.

Change-Id: I867118902abb2cf4b621e976b6ba953ef8f0431a
Reviewed-by: Daniel Teske <daniel.teske@theqtcompany.com>