Use standard QToolButtons so that their icons use the right colors

Summary:
BUG: 383059

For the inline context buttons, use the standard QToolButton widget which respects theming so that icons are always visible

Test Plan:
Tested in KDE Neon with Breeze light and dark, Oxygen, and all default color schemes. Big improvement.

Current with Breeze:

Patched version with Breeze:

Patched version with Breeze Dark:

Reviewers: KDE Applications, vtasoulas, broulik, rkflx

Reviewed By: rkflx

Subscribers: mart, rkflx

Differential Revision: https://phabricator.kde.org/D7988