[kcms/keyboard] tastenbrett: left-align key labels

When the horizontal alignment of a label is not specified, the default is to follow the natural alignment of the text, e.g. RTL text is aligned to the right, and LTR text is aligned to the left.

This causes key caps with labels with different directionality to look quite messed up.

Some examples:

Arabic (-l ara) before:

After:

Hebrew (-l il) before:

After:

Arabic basic extended (-l ara -a basic_ext) before:

After: