[applets/kickoff] Fix compact representation centering on specific sizes

Fixes drift toward bottom that may occur at specific applet sizes, such as with 64px tall panel and 150% global scaling on X11 for me.

Firstly, make the text label completely invisible when it has no text, thus avoiding extra spacing added by RowLayout.

Secondly, setting fontSizeMode to Text.VerticalFit makes sense only and
only if the component itself is constrained vertically, which is the
job for Layout.fillHeight property.

Lastly, set icon's Layout.alignment too, just in case.