v1: Backported A_Clipboard from alpha branch.

[jeeswg]

A built-in variable, A_Clipboard, equivalent to the existing built-in variable, Clipboard.
This makes AHK v1 consistent with AHK v2, so that both versions have A_Clipboard.

Test code:

;==================================================

;test A_Clipboard:

;manually: copy image data in MS Paint
vClipSaved := ClipboardAll

A_Clipboard := "abc"
MsgBox, % A_Clipboard "`r`n" Clipboard ;abc
A_Clipboard .= "def"
MsgBox, % A_Clipboard "`r`n" Clipboard ;abcdef

Clipboard := "ghi"
MsgBox, % A_Clipboard "`r`n" Clipboard ;ghi
Clipboard .= "jkl"
MsgBox, % A_Clipboard "`r`n" Clipboard ;ghijkl

A_Clipboard := vClipSaved
;manually: paste image data in MS Paint

;==================================================

[jeeswg]

A long-standing Clipboard append bug.

I discovered a bug in AHK v1 (since at least AHK 1.0.48.05).
If you select files in Explorer, and press Ctrl+C to copy, and then append text to the clipboard, e.g. Clipboard .= "abc", the resulting string is <<>>abc.

I discovered that the problem was fixed in AHK v2:
AutoHotkey v2.0-a110 had the problem.
AutoHotkey v2.0-a111 didn't.

This may be the relevant commit that fixed it, but it's a massive commit: 9d8f430f.

clipboard.h (currently identical in AHK v1 and v2) contains this line:
return IsClipboardFormatAvailable(CF_HDROP) ? _T("<<>>") : _T("");

(I've tried stepping through AHK v1. I don't expect to find a solution for weeks, due to some code that I'm not familiar with.)

This bug appears to be unrelated to a different bug that was fixed in v2.0-a101: 60d036a0.