fix a lot of low level cppcheck warnings (#9623)

* simplify the observer pattern, since all the called functions are const getters.
* use arduino macro over std: for numerical values and refactor local variables in drawScrollbar()
* oh, so Cppcheck actually complained about const pointers not being const.
* slowly getting out of ifdef hell
* fix inkHUD warnings as well
* last 2 check warnings
* git checks should fail on low defects from now on

.github/workflows/main_matrix.yml

@@ -95,7 +95,7 @@ jobs:
         with:
           pio_platform: ${{ matrix.check.platform }}
           pio_env: ${{ matrix.check.board }}
-          pio_target: check
+          pio_target: check --fail-on-defect=low
 
   build:
     needs: [setup, version]