[SystemTray] Fix item size for very narrow panel

Summary:
For very narrow panel item size is too big (item + padding exceed panel size). This adds a check to not exceed panel size.
Before:
{F8167941}
After:
{F8167942}

Test Plan:
* Make panel very narrow - icons should fit into panel
* Repeat for vertical/horizontal

Reviewers: #plasma_workspaces, #plasma, ngraham, broulik

Reviewed By: ngraham

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D27958