i18n: don't restrict LINGUAS at configure time

Don't use the $LINGUAS at configure time to set ALL_LINGUAS, and let
the translations Makefile properly honor the $LINGUAS itself, which it
already does better than we do, cleanly ignoring translations it
doesn't know.

If we do set ALL_LINGUAS=$LINGUAS, it will result in a build failure if
we do not have a translation for some of the language(s) specified in
$LINGUAS, and would make it impossible to build other languages without
re-configuring.  So, just drop that part and let the build-time support
kick in.

Closes #507.
Fix a named style misspelling in the Lua filetype
Clarified the location of the "Use project-based session files" option
Fix "Contributing to this document" for new HTML/PDF generation process
Sort FreeBasic keywords alphabetically
FreeBasic: Fix preprocessor keywords list

See http://www.freebasic.net/wiki/wikka.php?wakka=CatPgFunctIndex

Part of [feature-requests:#691]
FreeBasic: Update keywords

Patch from TJF.  Part of [feature-requests:#691]
Merge branch 'freebasic-ft-update'
Fix regexp search & replace.
Fix searching backwards from the Find dialog

search_find_prev() wasn't properly updated to work on GeanyFindFlags
leading to incorrect flags handling.
Remove unused flags from document_search_bar_find()

Passed-in flags was always 0, so the argument is not useful.
Also, this function expected Scintilla search flags rather than Geany
ones, making the API confusing for no good reason.
Use GeanyFindFlags instead of plain integer when expecting Geany find flags

This makes the API more obvious on what argument is expected, and avoid
confusion between Geany and Scintilla find flags.