code to change Windows-style backslashes in file paths to frontslashes

The task is to replace Windows backslashes in paths, without indiscriminately replacing backslashes in legitimate use as escapes (or bridge terrain).

Breaking this down:

* "no-syntax-rewrite": I don't think this is really necessary, but I will follow the practice of the hack_syntax section.
* if lines[i].lstrip().startswith("#"): Excludes lines that are only comments or defines.
* precomment: Originally, I simply excluded "#" during the while statement, but I realized that this could wrongly mistake a Pango color code (or old-style markup for green) as a comment. I now look for whitespace before "#", and rewrote this section to operate on precomment rather than the whole line.
* comment: Simply going with the second field would exclude the separator itself, so I use len.
* if '\\': Technically, this code worked going straight to the while statement, but running every line through the complicated regex made it sluggish. Faster to make sure the line meets a simple filter first.
* while: It is possible, though very uncommon, that a line could contain more than one file path. Looking at the regex test: a) Match the backslash itself, then be on the lookout for a character used to set apart a file path from other text. By excluding these, we ensure that there is an unbroken chain from the backslash to the file extension.  b) Then we have a list of file extensions, bracketed on the left by a period and the right by \b, to make sure they do not coincidentally match a string. These are the file types that might be referenced by a value (for instance, translation files are not referred to directly, so their extensions are not included). As a practical matter, EVERY instance in the wild I know of involves png, not counting one commented-out path in an ancient campaign.  c) File extensions can include capitals, particularly on Windows, where the effects of DOS unicase linger. So we make the search case-insensitive.
* regex object: This splits the line into groups. The differences from the regex used in the while statement: a) We also look for a non-pathbreaking string to the left of the backslash as well as the right. This means that group(1) will match the entire file path except the extension. b) The \b boundary has been made a non-consuming look-ahead assertion, to simplify future references to the regex object and its groups.
* fronted: The regex object, except all backslashes in group(1) are replaced by frontslashes.
* precomment: Here, we simply reconstruct precomment with the modified regex object.
* print: Besides reporting the substitution to stdout, I include a plea for cross-platform compatibility.
* lines[i]: Now it's time to rebuild the whole line.
regex sub to remove 'userdata/' from paths

The misguided authors who put userdata/ in their paths cause problems not just for non-Windows users, but fellow Windows users who chose not to put userdata in the install directory. This error can be removed by an approach similar to that just used to purge backslashes:

* if 'userdata': A basic filter to cut down the number of lines being run through complicated regex speeds up performance.
* while: It is possible, though rare, for a line to contain more than one path with userdata. Points about the regex: a) We continue to use precomment, though it would be well to correct commented-out old paths also, lest they mislead any more UMC writers.  b) In case you're wondering why I made one 'data/' string optional, there's a set of add-ons in 1.4 that use "userdata/campaigns" instead of "userdata/data/campaigns".  c) The '[ac]' at the end is something of an artifact of the time before I excluded comments, but it provides another safety measure insuring that the string is actually a value.
* regex object: This splits precomment into groups. Notes: a) Some authors begin with an unnecessary "../", might as well get rid of it as well. (As far as I can tell, this prefix has no effect anywhere I've seen it used, but I'd want to be positive that it ALWAYS does nothing before having wmllint replace it everywhere.) b) The first two groups have been made non-capturing; we will not need to refer to them.  c) For reporting to stdout, group(1) is extended to the next '/', though this part of the match is optional, to insure that there's no way to get trapped in a while loop.
* precomment: Here, we reconstruct precomment based on the regex object, except we simply drop what's outside group(1).
* print: In case designers don't get the point from seeing the elimination logged in stdout, I include an all-caps admonition against "userdata/". This is a really irritating bug.
* This code was inserted before the reconstruction of lines[i] from precomment and comment.