Display compiler versions (and, optionally, to switch between them)

[novusnota]

Observed behaviour

At the moment, one has to do a set of heuristics to determine which compiler version of FunC or Tact the project is using.

For example, to check if the compiler supports Tact v1.2.0, you may try to use += operator. And if that doesn't work, the version must be lower than 1.2.0.

Repeat this process to find the actual version used at the moment (or look into Nujan source code to find out faster, though still a bit impractical :)

Expected behavior or possible solutions

1. Display the currently used compiler version — it may be hidden under the cog icon of settings on the lower left.
2. Optionally, make it possible to switch between a pre-defined set of FunC/Tact versions.