Move internal struct from charset header to source

Also, rearrange and tidy the header.
Remove unused fields in CDDB dialog private struct
Updated French translation
Fix checking whether ID3v1 and ID3v2 are enabled

As reported by Steven Ulrick on the mailing list, saving files with ID3
tags and then reloading the directory causes the files to be shown as
modified, even though they are not.

https://mail.gnome.org/archives/easytag-list/2014-September/msg00014.html
Fix signal handler for apply-to-selection icons

Thanks to Steven Ulrick for the bug report.

https://bugzilla.gnome.org/show_bug.cgi?id=736315
Fix check for upper-casing prepositions

Flip the boolean check for whether to upper-case prepositions and other
excluded words.
Fix capitalization in process fields scanner

Iterate to the end of the string, rather than only processing the first
2 words.

https://bugzilla.gnome.org/show_bug.cgi?id=736609
Remove unused CDDB dialog private struct variable
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
Move scanner dialog to GtkBuilder
Move browser to GtkBuilder
Mark some strings as const in tagging functions
Use const for strings in tests and libapetag
Move file area to GtkBuilder
Move log area to GtkBuilder
Move tag area to GtkBuilder