qdoc: Allow example files to be excluded

Documentation configuration variables 'excludedirs' and 'excludefiles'
had no effect on what source and image files were listed as belonging
to an example.

This commit fixes that, and refactors excludedirs/files handling:
Remove duplicated code blocks, amd simplify the code reading the
variables:

  - Drop checks whether an excluded directory exists
  - Drop conversion of native separators for excluded paths

Both are unnecessary; QDoc expects '/'-separated paths for many other
variables, and if an excluded dir does not exist, it won't exclude
anything.

Change-Id: Ie25511e20d33888653c23bb84975368317eb4306
Reviewed-by: Alex Blasche <alexander.blasche@theqtcompany.com>
Reviewed-by: Martin Smith <martin.smith@theqtcompany.com>