[App] Use consistent unit schema names (#21355)

* [Start] ensure user's preferred unit schema is retained after restart of application

* [App] use consistent schema names throughout the application

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* [Start] Revert commit 2eea421

in preference to PR #21466

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

src/App/FreeCADInit.py

@@ -928,15 +928,16 @@ from enum import IntEnum
 # The values must match with that of the
 # C++ enum class UnitSystem
 class Scheme(IntEnum):
-    SI1 = 0
-    SI2 = 1
-    Imperial1 = 2
+    Internal = 0
+    MKS = 1
+    Imperial = 2
     ImperialDecimal = 3
-    Centimeters = 4
+    Centimeter = 4
     ImperialBuilding = 5
     MmMin = 6
     ImperialCivil = 7
-    FemMilliMeterNewton = 8
+    FEM = 8
+    MeterDecimal = 9
 
 App.Units.Scheme = Scheme