[Applet]Update layout based on T10470

Summary:
Decrease the Height of unknown connections.
Remove sections as headings, but keep available connections section as
a separator. Now positioned below active/deactivating (instead of only
connected) connections from the rest.
Readjust details expand/minimize area and highlight to the main row.
Set custom margins for ListItem's background.

Remove info row for unknown connections.
Move security type in details.
Add line between buttons and search field.
Change toolbar from GridLayout to ColumnLayout.
Add spacing between toolbuttons in toolbar.
Add opacity in details.

Test Plan:
Toolbar view with up-to-date build

Breeze Dark

Breeze Light

Air

Reviewers: Plasma, VDG, jgrulich, manueljlin, ngraham

Reviewed By: VDG, jgrulich, manueljlin, ngraham

Subscribers: ngraham, plasma-devel

Tags: Plasma

Maniphest Tasks: T10470

Differential Revision: https://phabricator.kde.org/D27199