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:
{F4047019}

Patched version with Breeze:
{F4103061}

Patched version with Breeze Dark:
{F4103078}

Reviewers: #kde_applications, vtasoulas, broulik, rkflx

Reviewed By: rkflx

Subscribers: mart, rkflx

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