Gitignore: "Convenience for devs" section

MantisBT devs frequently have to switch between older and newer
branches. Some files and dirs need to be excluded for convenience, and
also to prevent changes from being inadvertently checked in.

Regroup various file exclusions in a single "section" at the end of the
.gitignore file, to facilitate maintenance:
- v1.2.x config files (moved)
- REST API (new)
- Composer's 'vendor' directory (new)

.gitignore

@@ -1,11 +1,3 @@
-# Config files - v1.2.x config files - keep for convenience for dev machines switching branches
-config_inc.php
-custom_constants_inc.php
-custom_functions_inc.php
-custom_relationships_inc.php
-custom_strings_inc.php
-api/soap/mc_config_inc.php
-
 # Config files
 config/
 mantis_offline.php
@@ -49,3 +41,17 @@ web.config
 # Libraries
 library/FirePHPCore
 library/ezc
+
+# Settings below are for convenience when switching branches on Dev machines
+
+# v1.2.x config files
+config_inc.php
+custom_constants_inc.php
+custom_functions_inc.php
+custom_relationships_inc.php
+custom_strings_inc.php
+api/soap/mc_config_inc.php
+
+# REST API and Composer introduced in 2.3
+api/rest/
+vendor/