Attempt at implementing custom GUI controls

I've implemented a `Custom` control that can be used to easily add non-built-in controls to GUIs. It works like this (which is similar but not exactly your [original idea](http://www.autohotkey.com/board/topic/40320-/#entry251876)):

    DllCall("LoadLibrary", "str", "SciLexer.dll")
    Gui, Add, Custom, ClassScintilla ; creates a Scintilla control

Notes:
- The control has `WS_TABSTOP` by default.
- The 'Class' option must be specified on control creation
- The g-label is triggered when one of the following messages are received:
    - `WM_COMMAND`: A_GuiEvent is set to Normal and A_EventInfo is set to `HIWORD(wParam)` (according to MSDN it is a control-specified notification code).
    - `WM_NOTIFY`:  A_GuiEvent is set to N and A_EventInfo is set to the lParam (that is, the pointer to the `NMHDR` structure).
- GuiControl/Get just sets/retrieves the standard window text via Set/GetWindowText.

Further design input and bugfixes are very much appreciated.