infocomplete: Complete filetypes

[JoeKar]

This will add the capability to auto complete the filetype option.

Attention:
The duplicated entries will be solved with #3066.

Closes #3089

[JoeKar]

@dustdfg
It doesn't took that long. Maybe you can have a look next time, if you can provide the solution directly as PR without creating a new issue. 😄

[dustdfg]

@dustdfg It doesn't took that long. Maybe you can have a look next time, if you can provide the solution directly as PR without creating a new issue. 😄

You are absolutely right... But I:

1. Usually write spaghetti
2. Didn't look at auto-completion before at all
3. Didn't need it so bad
4. Knew about PR (Reduce the available string option validators and add autocompletion for them #3021)

I knew that the PR will likely invalidate my PR (if I decide to write it) if it will be merged so I decided just to create issue without attempts to create PR and even looking on that part of code...

One note: I even didn't look at code of that PR but the name sounds like it can conflict

[JoeKar]

One note: I even didn't look at code of that PR but the name sounds like it can conflict

Most probably OptionValueComplete() will need a small resolve, bu in general they don't really conflict each other, since #3021 tracks a specific kind of option predefined as list/choices. The filetype on the other hand is similar to the colorscheme, which uses stored filenames.