Replace Task Manager with Icons-Only-Task Manager in the default panel, and thicken it

Summary:
This patch implements the part of T12441 that was virtually universally agreed-to:
switching out the regular Task Manager for an Icons-Only Task Manager and pinning
some apps by default. The panel is also thickened to to 46px (with the default font,
since the calculation is based on GridUnits, which are font-aware). The panel remains
in its current position pending further discussion about whether or not we want to
move it to the left edge.

Ideally I'd like for this patch to used for discussions about the implementation, not the
general concept, so if you have concerns about that, please put them in T12441.

Test Plan: This is now the default panel: {F8290193}

Reviewers: #vdg, #plasma, niccolove, cblack

Reviewed By: #vdg, #plasma, niccolove, cblack

Subscribers: rikmills, mart, filipf, kmaterka, gvgeo, davidedmundson, ndavis, niccolove, The-Feren-OS-Dev, plasma-devel

Tags: #plasma

Maniphest Tasks: T12441

Differential Revision: https://phabricator.kde.org/D27845