Use string version number where applicable

The goal is to be able to use 'letter' releases such as '2.0.3a'
in the Version file.
SoObjects/Mailer/GNUmakefile.preamble: UIX_MAILER_*_VERSION are now strings
  Can't use SOGoVersion since the etag uses an 'underscore' notation (2_0_3a)
SoObjects/Mailer/*.m: updated format for UIX_MAILER_*_VERSION
SoObjects/SOGo/GNUmakefile: zap SOGo_VERSION, unsused
SoObjects/SOGo/GNUmakefile.preamble: fix defines
UI/MainUI/SOGoRootPage.m: use SOGoVersion

UI/MailerUI/UIxMailView.m

@@ -67,7 +67,7 @@ static NSString *mailETag = nil;
 
 + (void) initialize
 {
-  mailETag = [[NSString alloc] initWithFormat:@"\"imap4url_%d_%d_%03d\"",
+  mailETag = [[NSString alloc] initWithFormat:@"\"imap4url_%@_%@_%@\"",
                                SOGO_MAJOR_VERSION,
                                SOGO_MINOR_VERSION,
                                SOGO_SUBMINOR_VERSION];