keyboard: Prevent going forward before the localed proxy is ready

Otherwise we would crash on set_localed_input() and we really don't
want to move forward before we have a chance to pre-select the system
layout which we'll get from localed in the next patch.
keyboard: Ensure that we show the selected input source

This will become important in a coming commit that will pre-select the
system keyboard layout if one is set.
keyboard: Preserve system keyboard layouts

If there are keyboard layouts already configured system wide we should
keep and import them as user input sources.

For simplicity we still only allow the user to choose one input source
and in that case we'll use it as the default, i.e. the first.

The first system layout, if it exists, is pre-selected in the chooser.
keyboard: Don't choose irrelevant layouts as first level choices

If the language and country layouts aren't enough to fill our first
level number of rows we would end up showing totally irrelevant
layouts which looks bad and defeats the purpose of the initial
filtering.