The height of row set with `<row height="...">` cannot be lower than 0.5
to avoid rendering problems.
This restriction is not necessary when the row has no key and is lifted
in this case.

* Add <row scale=""> attribute

Scale the width of the keys in the row to match a value. Useful to
remove space on the right of the row without adding a 'width' attribute
to each key.

* layouts: Use the 'scale=""' attribute in builtin layouts

This removes the very specific 'width' values.

The number row option is changed into a ListPreference and controls whether the number row contains symbols or not.

Co-authored-by: Joey Schaff <j@jaoh.xyz>

* Selection mode: Space to cancel the selection

This adds the "selection mode", which is activated when text is selected
in the text box. The selection mode is exited when the selection is
cleared.

While the selection mode is activated, the Space and Esc keys are
modified into the "selection cancel" key, which remove the selection
without changing the text.
The space bar is otherwise easy to type by accident during a selection
and causes the selected text to be deleted.

* Selection mode: Move each ends of selection separately with slider

When the selection mode is activated, the space bar sliders change how
they affect the selection:

- The left side of the slider moves the left position of the selection.
  To shrink the selection from the left side, the slider must be
  activated by sliding to the left, extending the selection
  temporarilly, then by sliding to the right.

- The right side of the slider affects the right position if the
  selection.

Whether a key behaves as a slider is now purely defined by the key
values present on it.

The attribute 'c' specifies the symbol in the center of a key, just like
'key0'. This adds consistency with the cardinal direction attributes and
is intended to make layouts more readable.

This was causing a slight padding on the right of 4 columns layouts,
which turned into a large padding when the number row was added.