ViewerFrame: fixed two Checkstyle issues (mvn checkstyle:checkstyle)

ViewerFrame:
- Fixed two Checkstyle issues (after running "mvn checkstyle:checkstyle" and checking "target\checkstyle-result.xml").
- Used Utilities.setFontContainer (see below) to set the font for a group of components in the createReportIdPanel method.
- Removed unnecessary call to reportIdPanel.setForeground(Color.WHITE) from the createReportIdPanel method.

Constants:
- Made it more clear that REPORT_ERROR_LABEL_FONT is equal to DEFAULT_FONT.
- Removed some unnecessary "Constants." prefixes.

Utilities:
- Created the setFontContainer method to set the font for a container and all its components.