Properly unescape escaped characters in save file when loading

[ITdevkc]

Modified PRVM_ED_ParseEdict to determine whether it is loading a save or not, and unescape variables if loading a save. Fixes #81

[bones-was-here]

I changed this patch slightly because the exact cause of #81 is an incorrect special-case: all entity field values were unescaped when loading, consistent with what's done when saving, unless the name of the field was wad which has no equivalent special-case when saving.

That also caused a second bug: if a mapper set a field named anything other than wad to a value containing \ it was always treated as an escape when loading the BSP, even though map compilers don't support escapes (and thus fail to compile if " occurs in a field value). I had to work around this in Xonotic to support maps using entity sounds whose VFS path included \n.

I left the escape parsing in COM_ParseToken_Simple() (as already added in this PR) and disabled it entirely in PRVM_ED_ParseEpair() (where it was previously done for all fields except wad) thus fixing the second bug (and preventing double escape parsing). COM_ParseToken_Simple() already handles values containing } and escaped " whereas PRVM_ED_ParseEpair() doesn't.

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Compatibility notes: I tested some other engines and found they're too buggy for us to provide full save file compatibility: they don't escape when saving (meaning they can't support field values containing " even though those are valid in QC) but they do unescape when loading, which breaks field values containing \ such as that of world.wad on wickedstart.bsp.