Use a trash icon for anything that will remove a widget or a panel

Summary: Like D14116, but for the remaining uses of inappropriate close icons for buttons that will remove widgets or panels. We change those to trash icons to make it clear what will happen.

Test Plan:

Reviewers: broulik, Plasma

Reviewed By: broulik, Plasma

Subscribers: gregormi, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D14138