Add minimalist Wimp_ReadSysInfo 21 implementation

Since our text selection model follows the Style Guide and there's no means to configure it to the contrary, this subreason can just return fixed values to allow applications to query the state compared with running on a non-Cut-n-Paste Wimp.

hdr/Wimp

@@ -177,7 +177,7 @@ WimpSysInfo_ScrollPause         #       1               ; Wimp 3.99 onwards
 WimpSysInfo_Reserved_ROL18      #       1               ; RISC OS Select
 WimpSysInfo_Reserved_ROL19      #       1               ; RISC OS Select
 WimpSysInfo_Reserved_ROL20      #       1               ; RISC OS Select
-WimpSysInfo_Reserved_ROL21      #       1               ; RISC OS Select
+WimpSysInfo_TextSelection       #       1               ; Wimp 5.79 and RISC OS Select onwards
 WimpSysInfo_Reserved_ROL22      #       1               ; RISC OS Select
 WimpSysInfo_DragPause           #       1               ; Wimp 5.14 onwards
 WimpSysInfo_DoubleClickPause    #       1               ; Wimp 5.14 onwards
@@ -221,6 +221,19 @@ WimpTextOp_RightJustify         *       1 :SHL: 31
 WimpTextOp_VerticalJustify      *       1 :SHL: 30
 WimpTextOp_Style                *       1 :SHL: 29
 
+; Flags for Wimp_ReadSysInfo text selection
+
+WimpTextSelection_Enabled       *       1:SHL:0         ; Wimp CnP in writeables
+WimpTextSelection_EffectNOP     *       0
+WimpTextSelection_EffectDelete  *       1
+WimpTextSelection_EffectClear   *       2
+WimpTextSelection_EffectCut     *       3
+WimpTextSelection_EffectMask    *       2_11            ; 2 bits to describe the effect
+WimpTextSelection_InsertEffectShift *   1               ; Insert key and synonyms
+WimpTextSelection_DeleteEffectShift *   3               ; Delete key and synonyms
+WimpTextSelection_MoveEffectShift   *   5               ; Caret moved with cursors
+WimpTextSelection_AutoSelectEnabled *   1:SHL:7         ; Text is auto-selected when caret enters
+
 ; Priority for Wimp_CreateIcon (iconbar)
 
 WimpPriority_Maximum            *       &78000000       ; used for handle = -3 or -4

s/Wimp01

@@ -5523,7 +5523,7 @@ SWIWimp_ReadSysInfo
         B       err_badR0               ; 18= ?                        (ROL)
         B       err_badR0               ; 19= priority sprite area     (ROL)
         B       err_badR0               ; 20= special highlight colour (ROL)
-        B       err_badR0               ; 21= text selection behaviour (ROL)
+        B       sysinfo_textselection   ; 21= text selection behaviour
         B       err_badR0               ; 22= caret colour             (ROL)
         B       sysinfo_dragging        ; 23= mouse drag delay
         B       sysinfo_doubleclicks    ; 24= double click delay
@@ -5612,6 +5612,16 @@ sysinfo_IconBarInt
         STMIA   sp,{R1-R4}
         B       ExitWimp
 
+sysinfo_textselection
+        MOV     R0,#-1                  ; fg always from icon's own colours
+        MOV     R1,#-1                  ; bg always from icon's own colours
+        MOV     R2,#WimpTextSelection_Enabled :OR: \
+                    (WimpTextSelection_EffectDelete:SHL:WimpTextSelection_InsertEffectShift) :OR: \
+                    (WimpTextSelection_EffectCut:SHL:WimpTextSelection_DeleteEffectShift) :OR: \
+                    (WimpTextSelection_EffectClear:SHL:WimpTextSelection_MoveEffectShift)
+        STMIA   sp,{R1-R2}
+        B       ExitWimp
+
 sysinfo_dragging
         LDRB    R0,drag_movelimit
         LDR     R1,drag_timelimit