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>