applets/kickoff: Reiterate on compact representation sizing code, add fallback icon
All 4 min/max width/height expressions were combined into one property block for deduplication and code locality. Some helper properties were introduced, others removed as obsolete.
Compact icon now does not shrink to a size less than a panel's orthogonal direction, such that is stays at least square or taller (in vertical) / wider (in horizontal form factor).
Extra margins are added to the left and right of the label (if present) instead of spacing between it and an icon. Otherwise text looks too sticky to its neighboring widgets IMHO.
Test plan: check with and without icons, text on horizontal, vertical panels and on a desktop; in every form factor use icons of different aspect ratio: square, landscape, portrait, and an invalid one (can be set to a valid, then move the icon file and restart plasmashell).
Bonus patch:
applets/kickoff: Fallback to a default icon if the requested one fails to load
It may so happen that you set a custom icon, then moved icon file elsewhere, and upon next login your Kickoff Application Launcher widget would become practically invisible and unclikcably small.
Also drop smooth initialization: it is true by default anyway.