Ability to define backpack radios in config.

Example
```
tf_hasLRradio = 1;
tf_side = "West";
tf_dialog = "rt1523g_radio_dialog";
```
This will allow mods to create their own backpack radios with their own interface (procedure to do so will need to be documented).

Unsure if support will be required from plugin side.

This is to move in the direction of supporting the ability for mods to provide their own radios.