Add schematic for a constant voltage source

symbols/spannungsquelle.dcm

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+EESchema-DOCLIB  Version 2.0
+#
+#End Doc Library

symbols/spannungsquelle.lib

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+EESchema-LIBRARY Version 2.3
+#encoding utf-8
+#
+# Spannungsquelle
+#
+DEF Spannungsquelle U 0 0 N Y 1 F N
+F0 "U" 100 50 50 H V L CNN
+F1 "Spannungsquelle" 100 -50 50 H V L CNN
+F2 "" 0 10 50 V V C CNN
+F3 "" 0 10 50 V V C CNN
+DRAW
+S -90 20 90 10 0 1 0 F
+S -43 -12 44 -22 0 1 0 F
+P 2 0 1 10  20 65  60 65 N
+P 2 0 1 10  40 85  40 45 N
+X ~ 1 0 150 134 D 50 50 1 1 P
+X ~ 2 0 -150 134 U 50 50 1 1 P
+ENDDRAW
+ENDDEF
+#
+#End Library