Revert "Disallow specifying file paths with backslashes"

This reverts commit 1b9c07e.

I'm reverting this (at least temporarily) since it seems mainline contains *numerous* paths with
backslashes. This change basically spams console with warnings just from mainline/core content,
in addition to having other negative side effects such as leader image generation failing when
populating a save_index file. Plus, if backslashes really broke things on Linux, I think we'd have
had a multitude of people complaining about half their images not showing up on that platform, which
(AFAIK) has not been the case.

RELEASE_NOTES

@@ -16,24 +16,30 @@ CHANGES
 
 [rasection="Campaigns"]
 [list]
+[*]The order of the beginner campaigns is now [i]A Tale of Two Brothers, An Orcish Incursion, The South Guard, & Heir to the Throne.[/i] It is hoped that this sequence will be friendlier to new players, encouraging them to start with the simpler AToTB, so they might be better prepared to enjoy HttT.
 [/list]
 [/rasection]
 
 [rasection="User Interface"]
 [list]
+[*]Most text boxes now support advanced Input Method Editors (in-game chat does not, however).
 [/list]
 [/rasection]
 
 
 [rawarn="Deprecations and breaking changes"]
 [list]
-[*] File paths with backslashes are no longer accepted. Use forward slashes instead.
+[*] moveto and enter_hex, exit_hex event no longer abort the movement, [cancel_action] must be used to cancel the current move.
+[*] The allow_new_game= attribute in [scenario] now defaults to false (it still defaults to tru in [multiplayer]) 
+[*] In gui2 dialogs/widgets [resolution] window_width/height now specifies the minimum window size for that resolution ot be chosen 
 [/list]
 [/rawarn]
 
 
  [rasection="New lua/wml features"]
  [list]
+ [*] formula code is now supported in abilities and weapon specials
+ [*] [multiplayer]/[scenario] now support mp_village_gold, mp_village_support, mp_fog and mp_shroud keys, which are useed as default values for the corresponding keys in [side], it is reccomended to use them instead of the keys in [side] for configurable multiplayer scenarios.
  [/list]
  [/rasection]
 

changelog

@@ -4,9 +4,6 @@ Version 1.13.10+dev:
  * User Interface:
    * Removed broken Unit Box and Widescreen themes.
  * WML Engine:
-   * File paths with backslashes are no longer allowed. This ensures that a UMC
-     author can't accidentally use them and make an add-on that breaks on
-     GNU/Linux and macOS.
    * File paths are now case sensitive even on Windows.
 
 Version 1.13.10:

src/filesystem_boost.cpp

@@ -1147,11 +1147,6 @@ static bool is_legal_file(const std::string &filename)
 		return false;
 	}
 
-	if (filename.find('\\') != std::string::npos) {
-		ERR_FS << "Illegal path '" << filename << R"end(' ("\" not allowed, for compatibility with GNU/Linux and macOS).)end" << std::endl;
-		return false;
-	}
-
 	if (looks_like_pbl(filename)) {
 		ERR_FS << "Illegal path '" << filename << "' (.pbl files are not allowed)." << std::endl;
 		return false;