applets/kickoff: Improve adaptive layout of the Footer/LeaveButtons

It used to overflow because it did not account for the width of the overflow menu button when estimating the width required for the LeaveButtons layout as a whole. Now not only it does account for the overflow menu button, but the button itself also changes display modes (technically swapping between two mutually exclusive buttons at most one of which would be visible at any given time) depending on the available space. The whole layout calculation is done in a centralized place and results in an atomic update of var typed property (JS object) with four parameters.

Requires libplasma!1165 (merged)