InputText: use TextSrc more consistently to facilitate accessing user buffer in text processing code. (#8242)

Followup to e900571
Removed SetClipboardText() trick used in abd07f6d (#7925)

imgui_internal.h

@@ -1124,6 +1124,7 @@ struct IMGUI_API ImGuiInputTextState
     ImGuiInputTextFlags     Flags;                  // copy of InputText() flags. may be used to check if e.g. ImGuiInputTextFlags_Password is set.
     ImGuiID                 ID;                     // widget id owning the text state
     int                     TextLen;                // UTF-8 length of the string in TextA (in bytes)
+    const char*             TextSrc;                // == TextA.Data unless read-only, in which case == buf passed to InputText(). Field only set and valid _inside_ the call InputText() call.
     ImVector<char>          TextA;                  // main UTF8 buffer. TextA.Size is a buffer size! Should always be >= buf_size passed by user (and of course >= CurLenA + 1).
     ImVector<char>          TextToRevertTo;         // value to revert to when pressing Escape = backup of end-user buffer at the time of focus (in UTF-8, unaltered)
     ImVector<char>          CallbackTextBackup;     // temporary storage for callback to support automatic reconcile of undo-stack