Initial hotpluggable keyboard handling (#9540)

* Added a new plugin external-keyboard. It listens to USB events. When keyboard is plugged in or plugged out, it updates device and input configuration accordingly.
* Added new fake events UsbDevicePlugIn and UsbDevicePlugOut that are emitted when a device is connected to a book reader that plays the role of USB host. The usage of the existing events UsbPlugIn and UsbPlugOut has not changed - they are used when a reader is connected to a host. The koreader-base has a related PR for those events.
* Did a small refactoring of initialization for the modules FocusManager and InputText. They check device keyboard capabilities on their when the module is first loaded and store it. Some of the initialization code has been extracted into functions, so that we can re-initialize them when keyboard is (dis)connected.
* Initial implementation centered around text input, and tested with USB keyboards on devices with OTG support.
* Said OTG shenanigans are so far supported on devices with debugfs & the chipidea driver, or sunxi devices.

frontend/ui/widget/inputtext.lua

@@ -72,9 +72,7 @@ function InputText:initEventListener() end
 function InputText:onFocus() end
 function InputText:onUnfocus() end
 
--- only use PhysicalKeyboard if the device does not have touch screen
-if Device:isTouchDevice() or Device:hasDPad() then
-    Keyboard = require("ui/widget/virtualkeyboard")
+local function initTouchEvents()
     if Device:isTouchDevice() then
         function InputText:initEventListener()
             self.ges_events = {
@@ -284,6 +282,9 @@ if Device:isTouchDevice() or Device:hasDPad() then
             return false
         end
     end
+end
+
+local function initDPadEvents()
     if Device:hasDPad() then
         function InputText:onFocus()
             -- Event called by the focusmanager
@@ -302,10 +303,23 @@ if Device:isTouchDevice() or Device:hasDPad() then
             return true
         end
     end
-else
-    Keyboard = require("ui/widget/physicalkeyboard")
 end
 
+-- only use PhysicalKeyboard if the device does not support touch input
+function InputText.initInputEvents()
+    FocusManagerInstance = nil
+
+    if Device:isTouchDevice() or Device:hasDPad() then
+        Keyboard = require("ui/widget/virtualkeyboard")
+        initTouchEvents()
+        initDPadEvents()
+    else
+        Keyboard = require("ui/widget/physicalkeyboard")
+    end
+end
+
+InputText.initInputEvents()
+
 function InputText:checkTextEditability()
     -- The split of the 'text' string to a table of utf8 chars may not be
     -- reversible to the same string, if 'text'  comes from a binary file