The current approach is hard to maintain, for example the last key
"autofill" was not displayed.

This implements a PreferenceGroup that contains the check boxes for
every extra keys without involving listing the preferences in
settings.xml.

A custom layout is used to remove the 'title' text view.
The list of extra keys is moved into the new class.
'ExtraKeyCheckBoxPreference' becomes a nested class.

The new key switches to any installed "voice" input method.
If several input methods matches, no effort is made to choose.
Might misbehave with some input methods other than Google's on API < 28.

It is placed on the middle of the arrows on the bottom bar. It is
enabled by default and can be removed in the "Extra keys" option.

The key is not removed from the keyboard if no voice input method
exists.

Added missing accent (horn, hook, dot below)
Added layout
Added accent font svg (by modified existing accent)

The most requested keys are undo and redo. Unfortunatly redo doesn't
work reliably.

The other context menu actions like share, assist and autofill are added
even thought they are rarely useful or implemented.

The 'accent_slash' was missing in the extra keys setting.

Allow to type more latin letters "with oblique stroke" or "with stroke"
that visually have an oblique bar, that were not added to 'accent_bar'.

Allows to type many of the "with stroke" or "with bar" latin letters.

The Meta key is not useful.
The Alt key can be used in a terminal but is not useful to a part of
terminal users.

The symbols alone might be hard to understand when scrolling through the
"extra keys" option.

The key enable caps lock immediately. It does nothing if caps lock is
already enabled.

It is not present on the keyboard by default but a place is defined on
every layout, top-right of the shift key. It can be enabled in the
settings.

The icon is from materialdesignicons.com.