Remove redundant NULL checks in CDDB dialog

Found with Coverity (CID 1238682, CID 1238683, CID 1238684, CID 1238685,
CID 1238686)
Remove dead code in et_browser_label_set_text()

Found with Coverity (CID 1238690).
Use return value to speed up force saving files

The variable assignment was lost during refactoring.

Found with Coverity (CID 1238689).
Move remaining upper-casing functions to scan.c

Split out Scan_Process_Fields_First_Letters_Uppercase() and
Scan_Word_Is_Roman_Numeral() and add some parameters, so that they do
not depend on the state of GSettings keys internally.
Add simple tests for style-sensitive case function
WIP Move scanner dialog to GtkBuilder

TODO: shrink dialogue vertically, handle legend and mask editor better,
handle disparate dimensions of fill/rename and process scanner notebook
pages