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/settings/SettingsActivity.cpp

@@ -21,7 +21,7 @@ const SettingInfo settingsList[settingsCount] = {
     SettingInfo::Enum("Status Bar", &CrossPointSettings::statusBar, {"None", "No Progress", "Full"}),
     SettingInfo::Toggle("Extra Paragraph Spacing", &CrossPointSettings::extraParagraphSpacing),
     SettingInfo::Toggle("Text Anti-Aliasing", &CrossPointSettings::textAntiAliasing),
-    SettingInfo::Toggle("Short Power Button Click", &CrossPointSettings::shortPwrBtn),
+    SettingInfo::Enum("Short Power Button Click", &CrossPointSettings::shortPwrBtn, {"Ignore", "Sleep", "Page Turn"}),
     SettingInfo::Enum("Reading Orientation", &CrossPointSettings::orientation,
                       {"Portrait", "Landscape CW", "Inverted", "Landscape CCW"}),
     SettingInfo::Enum("Front Button Layout", &CrossPointSettings::frontButtonLayout,