[BUGFIX] ColorPicker: Use absolute web path for exampleImg

The Example Image of the color picker is currently hardcoded (also with ..)
to the typo3/ directory.
This hinders integrators to allow any other file outside of typo3/ to be shown.

The patch corrects the path resolving part of the Color Picker.

Resolves: #70428
Releases: master
Change-Id: I2fb5a6b7379ee1cb5fd6e76d8081f2f72f91a098
Reviewed-on: https://review.typo3.org/45140


Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Andreas Fernandez <typo3@scripting-base.de>
Tested-by: Andreas Fernandez <typo3@scripting-base.de>
Reviewed-by: Morton Jonuschat <m.jonuschat@mojocode.de>
Tested-by: Morton Jonuschat <m.jonuschat@mojocode.de>