GitLab

Detail:
* Rewrite `Makefile` to use shared makefile fragments
* No longer requires `-E` switch to `amu`
* Consolidate `.gitignore` files

Admin:
  Requires BuildSys-7_50 and RISC_OSLib-6_08.

Version 1.75. Tagged as 'EditApp-1_75'

edit.c:
txtar.h and txtoptmenu.h weren't actually needed since none of their definitions were in use.
txtfile.h was being used for a single function which is now implemented in save_one_txt(), sharing code with the panic file save function.
Accept RISC_OS_PLUS switch (circa RISC OS 3.00) is always true now, and delete some experimental code.
Fix bug in panic file saves to Wimp$ScrapDir, the filetype was being derived from bits 20-31 of the load address rather than 8-19. Panic save also changed to use RISC_OSLib style SWI functions rather than _swi and _swix.
message.c:
Take ownership of the task window defines, same as SrcEdit has for some years.

Tested in a RAM install, printing a 2 line BASIC program out.

Version 1.70. Tagged as 'Edit-1_70'