Add page turn on power button press (#286)

## Summary

* **What is the goal of this PR?** 
* This PR adds a setting to (additionally) map the forward page turn
onto the powerbutton when in `EPUBReaderActivity` and powerbutton short
press is not mapped to sleep mode. I find the powerbutton to be exactly
where my thumb is while reading so it is very convenient to map the
forwardpage turn to that. Maybe Im not alone with this ^^
* **What changes are included?**

## Additional Context

* Add any other information that might be helpful for the reviewer
(e.g., performance implications, potential risks, specific areas to
focus on).

src/activities/reader/EpubReaderActivity.cpp

@@ -152,6 +152,8 @@ void EpubReaderActivity::loop() {
   const bool prevReleased = mappedInput.wasReleased(MappedInputManager::Button::PageBack) ||
                             mappedInput.wasReleased(MappedInputManager::Button::Left);
   const bool nextReleased = mappedInput.wasReleased(MappedInputManager::Button::PageForward) ||
+                            (SETTINGS.shortPwrBtn == CrossPointSettings::SHORT_PWRBTN::PAGE_TURN &&
+                             mappedInput.wasReleased(MappedInputManager::Button::Power)) ||
                             mappedInput.wasReleased(MappedInputManager::Button::Right);
 
   if (!prevReleased && !nextReleased) {

src/activities/reader/XtcReaderActivity.cpp

@@ -112,6 +112,8 @@ void XtcReaderActivity::loop() {
   const bool prevReleased = mappedInput.wasReleased(MappedInputManager::Button::PageBack) ||
                             mappedInput.wasReleased(MappedInputManager::Button::Left);
   const bool nextReleased = mappedInput.wasReleased(MappedInputManager::Button::PageForward) ||
+                            (SETTINGS.shortPwrBtn == CrossPointSettings::SHORT_PWRBTN::PAGE_TURN &&
+                             mappedInput.wasReleased(MappedInputManager::Button::Power)) ||
                             mappedInput.wasReleased(MappedInputManager::Button::Right);
 
   if (!prevReleased && !nextReleased) {