QColorDialog: Fix a few layouting issues.

Fixes combo-boxes being too big and some alignment issues
that were not present in Qt4.

See screenshot in JIRA task.
Tested with QT_SMALL_COLORDIALOG too.

Task-number: QTBUG-43501
Change-Id: I2aefb64be1c5f3c4894149d85e1a12c9c0cc3d62
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>