Enforce Breeze icon theme for all icons

Without this, on Windows, Kleopatra used a mix of Breeze icons and icons
provided by QWindowsIconEngine. In high-contrast mode this caused
inconsistent coloring of disabled icons where Breeze icons were greyed
out (like the icons of disabled actions in Windows Explorer) while the
icons provided by QWindowsIconEngine where colored with the color for
disabled text.

• GnuPG-bug-id: T7588