GitLab

Modernise build structure

Detail:
* Rewrite `Makefile` to use shared makefile fragments
* Uses `modgen` to create the !Alarm module, which obsoletes some source files
* Rename a couple of resource files to match their names when installed
* All the files in `Resources:$.Apps.!Alarm` are now registered by the !Alarm
  module, so you can kill or unplug the module to hide it
* The `Code` blob used to be installed in ResourceFS both by the Messages
  module and by the !Alarm module - now de-duplicated (only in Messages)
* Consolidate `.gitignore` files
* Enable GitLab CI

Admin:
  Requires RiscOS/BuildSys!32