Fix enabled state of select next/previous element button in tools floater

indra/newview/llfloatertools.cpp

@@ -879,6 +879,11 @@ void LLFloaterTools::updatePopup(LLCoordGL center, MASK mask)
 	// <FS:PP> FIRE-14493: Buttons to cycle through linkset
 	mBtnPrevPart->setVisible(edit_visible);
 	mBtnNextPart->setVisible(edit_visible);
+
+	bool select_btn_enabled = (!LLSelectMgr::getInstance()->getSelection()->isEmpty()
+								&& (linked_parts || LLToolFace::getInstance() == LLToolMgr::getInstance()->getCurrentTool()));
+	mBtnPrevPart->setEnabled(select_btn_enabled);
+	mBtnNextPart->setEnabled(select_btn_enabled);
 	// </FS:PP>
 
 	if (mCheckSelectIndividual)

indra/newview/skins/default/xui/en/floater_tools.xml

@@ -282,7 +282,6 @@
 	</check_box>
 
    <button
-     enabled_control="EditLinkedParts"
      follows="left|top"
      height="20"
      image_overlay="BackArrow_Off"
@@ -296,7 +295,6 @@
 	     parameter="previous" />
     </button>
    <button
-     enabled_control="EditLinkedParts"
      follows="left|top"
      height="20"
      image_overlay="ForwardArrow_Off"

indra/newview/skins/starlight/xui/en/floater_tools.xml

@@ -280,7 +280,6 @@
 	</check_box>
 
    <button
-     enabled_control="EditLinkedParts"
      follows="left|top"
      height="20"
      image_overlay="BackArrow_Off"
@@ -294,7 +293,6 @@
 	     parameter="previous" />
     </button>
    <button
-     enabled_control="EditLinkedParts"
      follows="left|top"
      height="20"
      image_overlay="ForwardArrow_Off"

indra/newview/skins/starlightcui/xui/en/floater_tools.xml

@@ -280,7 +280,6 @@
 	</check_box>
 
    <button
-     enabled_control="EditLinkedParts"
      follows="left|top"
      height="20"
      image_overlay="BackArrow_Off"
@@ -294,7 +293,6 @@
 	     parameter="previous" />
     </button>
    <button
-     enabled_control="EditLinkedParts"
      follows="left|top"
      height="20"
      image_overlay="ForwardArrow_Off"

indra/newview/skins/vintage/xui/en/floater_tools.xml

@@ -282,7 +282,6 @@
 	</check_box>
 
    <button
-     enabled_control="EditLinkedParts"
      follows="left|top"
      height="20"
      image_overlay="BackArrow_Off"
@@ -296,7 +295,6 @@
 	     parameter="previous" />
     </button>
    <button
-     enabled_control="EditLinkedParts"
      follows="left|top"
      height="20"
      image_overlay="ForwardArrow_Off"