Keep startup notifications launcher-sized when inserting at launcher position.

Summary:
When "Keep launchers seperate" is disabled, startup notifications
replace launchers at their original position. This changes the
layout for this mode to keep the startup notification item the size
of the launcher item it replaces (the visual result being a throbber
on the launcher instead of the launcher being replaced by a "task"
with a throbber - iow, the expansion to task size is delayed until
a window appears).

This helps the minimize the jarring visual fallout in cases where
we can match the startup notification to the launcher, but not the
window to the startup notification (due to broken/missing window
metadata), as no longer will two task-sized items co-exist - instead
the throbber comes and goes on the launcher, seperate from the task
item for the unidentified window. It also minimizes visual fallout
if app startup fails or the app merely creates an SNI instead of a
window.

This change was suggested by Netrunner Linux, which ships a Task
Manager with "Keep launchers seperate" disabled by default.

Depends on D5504.

Reviewers: #plasma, davidedmundson

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D5505