Modernise build structure

* Cross-compilable
* `Resources` subdirectory reorganised into more standardised structure -
  permits installation using `InstRes` and should make it more obvious how
  to modify the set of resource files in future
* Changed to use `ResGen` for standalone builds (which requires changing to
  use `CModule` shared makefile)
* Resources made consistent between softload and ROM builds (so for example,
  the "Zip" sprites for softload SCSIFiler builds are retired)
* No attempt has been made to retain merging of `CmdHelp` files, since we
  don't use any in this component - this simplifies things
* `IfThere` removed from `install` rule for `ThemeDefs` targets - all UserIF
  setting appear to have sprites available, so this appears to no longer be
  required, and its absence simplifies cross-compiling support
* `debug` target now requires amu 5.35 (saves having to write out a separate
  rule for cross-compiling case, but has no impact on any other target)
* `ASFLAGS` are now inferred from `COMPONENT`, so could be removed from
  Component files in future if desired
* Consolidate and complete `.gitignore`
* Enable GitLab CI
12 jobs for !5 with CrossCompilationSupport in 19 seconds (queued for 35 seconds)
latest detached
Status Job ID Name Coverage
  Static Analysis
passed #63441
cross
copyright

00:00:00

passed #63432
cross
gitattributes

00:00:01

passed #63433
cross
gitignore

00:00:00

passed #63436
cross
head_log

00:00:00

passed #63439
cross
head_whitesp

00:00:00

passed #63434
cross
license

00:00:00

passed #63438
cross
makefile

00:00:00

passed #63437
cross
merge_log

00:00:01

passed #63440
cross
merge_whitesp

00:00:01

passed #63435
cross
versionnum

00:00:00

 
  Softload
passed #63442
cross
softload

00:00:07

 
  Cleanup
passed #63443
cross
cleanup

00:00:02