Many uses of RGBM instead of RGBA

[gab3d]

In the TIF file format Options dialog, in the Bits per pixel option it reads RGBM instead of RGBA:

• 32(RGBM)
• 64(RGBM)

[RodneyBaker]

Informational:

Here's a link to all the instances of the term RGBM both cosmetic and programmatic in the code:

https://github.com/opentoonz/opentoonz/search?q=RGBM&unscoped_q=RGBM

See earlier PRs for when/why RGBM was changed to RGBA.

[RodneyBaker]

As we are heading into v1.4 release it will be good to let all translators know to look for instances of "RGBM" in the .ts files and adjust those to "RGBA". That alone may resolve many appearances of the older abbreviation in the UI.

I'm not convinced that we want to change RGBM to RGBA in all instances of the code source quite yet although I'm sure a full scrub/replacement of every instance would be fine.