Fix variable scopes (static), Simplify encoder

- add static type to all local variables in every source file
  so the scope of each variable is limited to that file.
  This prevents conflicting variable names with other source file
  (e.g. 'state' variable in vfd.cpp)

- simplify encoder concept
  wrap all used functions from rotary_encoder.h library in
  custom encoder.hpp file
   -> only one file has to be included where encoder is used
   -> global variable 'encoder' is not necessary anymore
   -> reduces duplicate code for length calculation
- update all files where encoder functions where used accordingly

main/vfd.cpp

@@ -4,9 +4,9 @@
 
 const char* vfd_directionStr[2] = {"FWD", "REV"};
 static const char *TAG = "vfd";
-uint8_t level = 0; //current speed level
-bool state = false; //current state
-vfd_direction_t direction = FWD; //current direction
+static uint8_t level = 0; //current speed level
+static bool state = false; //current state
+static vfd_direction_t direction = FWD; //current direction
 
 
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