Common linters by default in the Build menu

This is a start of the effort to provide a bit more integrated development solution. Eclipse, Vim, Sublime, etc. modern IDE/editors allow for common linters to highlight erroneous and _smelly_ source code lines, some even on-the-fly in the marker margin.

Providing the configured default linter options in `data/filetype.*` files is a cheap and easy hack achieving somewhat similar functionality. The user still needs to have installed the linters they want to use. As discussed with @b4n on IRC, this is in order.

I did my best to research the most common linter packages for a particular language, opting for nothing in particular where there are variety of options with lack of consensus.

Additionally, I migrated old-style `[build_settings]` config blocks to new-style `[build-menu]` in a separate commit. Supposedly, @elextr or someone besides @b4n has to confirm that `build-menu.FT_00_CM` won't override user's custom set `build_settings.compiler` or similar. Be that the case, I can happily migrate `compiler`/`linker`/`run_cmd` commands in other filedef files as well.
There is also PyLint which is pretty popular, but let's say PEP-8 is The Style Guide for Python code and will satisfy >90% of users.

Also on max-line-length, it's pretty popular today with wider screens to have a bit wider lines. `pep8` tool doesn't provide a stand-alone switch for no reason. There's already a long line marker in Geany, and 120 is only a sane default.