i18n: mark strings in code for translation

Marks all string in the code to be translated with gettext macros.
POTFILES.in is updated to only contain source files with translatable strings.
po/README instructions are updated so that xgettext finds all the used macros.

C_() is used to give context to a translatable string.

Issue: #78928
Change-Id: I27bd91ff12a9913ac0d41842e449daf73197d96b
Signed-off-by: Guillaume Roguez <guillaume.roguez@savoirfairelinux.com>
diff --git a/po/POTFILES.in b/po/POTFILES.in
index 44710f5..a079c65 100644
--- a/po/POTFILES.in
+++ b/po/POTFILES.in
@@ -1,29 +1,22 @@
 # List of source files containing translatable strings.
 # Please keep this file sorted alphabetically.
 
-src/accountadvancedtab.cpp
 src/accountaudiotab.cpp
 src/accountgeneraltab.cpp
-src/accountsecuritytab.cpp
 src/accountvideotab.cpp
 src/accountview.cpp
 src/callsview.cpp
-src/choosecontactview.cpp
-src/contactpopover.cpp
 src/contactsview.cpp
-src/currentcallview.cpp
 src/dialogs.c
 src/editcontactview.cpp
 src/frequentcontactsview.cpp
 src/generalsettingsview.cpp
 src/historyview.cpp
-src/incomingcallview.cpp
-src/mediasettingsview.cpp
-src/ringmainwindow.cpp
 src/ringnotify.cpp
 src/ring_client_options.c
 src/backends/edscontactbackend.cpp
 src/utils/menus.cpp
+src/ringmainwindow.cpp
 ui/accountadvancedtab.ui
 ui/accountaudiotab.ui
 ui/accountgeneraltab.ui