Work around clearing the output panel when setting a regexp

This will make the output for converting more straight forward.
Preview the file name (mainly extension) in the target_format prompt
Do fail highlighting a match pattern with '#' as the first character

The appropriate thing to do is probably highlighting this as a comment but
this way is so much easier.
Add a not really working rule to Oniguruma RegExp for (?x) extended groups

This was supposed to detect # comments after (?x) has been enabled but it is
not possible to do this throughout the whole "string" (terminated by the first
end-match of the parent match which included the match in question, for example).
Thus, this will only work till the end of the line in which (?x) occurred.
Oniguruma: "effect range of isolated option is to next ')'."
Revamp YAML syntax def a bit

- Update a few comments
- Detect comments in multi-line "fileTypes" array
- Detect various multi-line comment scenarios (block, quoted or even unquoted)