Fix pixel alignment and non-standard height in QuickShare delegate

Summary:
The current code had height: button.height * 1.2
then centered the button within it, which can result in non-pixel
aligned y values, leading to rendering issues.

We don't need to specify the height explicitly, it's done by ListItem if
we fix the underlying binding loop. This gives the delegate the correct
size of button height + margins.

BUG: 375137

Reviewers: #plasma

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D4166