The touch-keyboard component presents a virtual keyboard on screen and emits
input events when the user touches the virtual keys. It requires a GUI service,
a "fonts" file-system session, and an event session.

By default, the keyboard is positioned at the top-left corner of the screen
with the smallest possible size, given the used font. Those defaults can be
the overridden by the configuration as follows.

! <config min_width="720" min_height="360" xpos="10" ypos="10"
!         opaque="yes" background="#000000"/>

The 'opaque' and 'background' attributes control the appearance of the
background. When setting opaque to "yes", the alpha channel is disabled
and the color specified via the 'background' attribute is applied.

The layout of the virtual keyboard is defined by a ROM module requested via
the label "layout". An example can be found at
_recipes/raw/touch_keyboard/touch_keyboard_layout.config_.
