Rename OMNI and TOMNI to OMN and TOMN

[dexX7]

To be more distinctive to other projects, the following changes in wording are made:

• "Omni", referring to the native tokens of the Omni Layer protocol, becomes "Omni tokens"
• "Test Omni", referring to the native test tokens of the Omni Layer protocol, becomes "Test Omni tokens"
• "OMNI", referring to the symbol of Omni tokens, becomes "OMN"
• "TOMNI", referring to the symbol of Test Omni tokens, becomes "TOMN"

While this is change is mostly cosmetic - in particular it changes the code documentation, RPC help messages and RPC documentation - it also has an effect of the RPCS "omni_getproperty 1" and omni_getproperty 2", which now return a text with the updated token and symbol names.

[dexX7]

TODO: update OmniJ, resubmit to dev branch

[dexX7]

Merged via fa45eb6.