By Brad Brooks UVALDE, Texas (Reuters) – The Texas gunman who murdered 19 children and two teachers posted on Facebook that he was going to shoot up an elementary school about 15 minutes before his rampage, Governor Greg Abbott said on Wednesday, as harrowing details about the attack continued to emerge. The gunman, identified as 18-year-old Salvador Ramos, also posted a message saying he was going to shoot his grandmother, as well as another one confirming that he had done so, Abbott said at a news conference. His grandmother, whom Ramos shot in the face shortly before attacking the school, s…