build/windows: Organize *gimp3264.iss [Code] to not look like Assembly

The installer is probably the most complex packaging format so one of the
biggest examples of packaging love over the years into it in our repo.
But the readability of installer scripts... leave a lot to be desired.

Now, the main script is organized following the order of installer pages
with some comments. This will make easier to future contributors work
(but this commit probably isn't perfect since it's a big change to read)

build/windows/installer/util_rebootcontinue.isi

@@ -20,6 +20,8 @@ begin
 	Result := S + ' /' + P;
 end;
 
+const
+	RunOnceName = 'Resume GIMP {#GIMP_VERSION} install';
 
 procedure CreateRunOnceEntry;
 var	RunOnceData, SetupRestartData: String;

build/windows/installer/util_uninst.isi

@@ -2,6 +2,9 @@
 [Code]
 #endif
 
+var
+	asUninstInf: TArrayOfString; //uninst.inf contents (loaded at start of uninstall, executed at the end)
+
 function SplitRegParams(const pInf: String; var oRootKey: Integer; var oKey,oValue: String): Boolean;
 var	sRootKey: String;
 	d: Integer;