Summary:
This implements the menu shadow changes proposed in D9549; now we use a combobox with five options: None, Small, Medium, Large, and Very Large.

Large is the new default value (64px window shadows, 16px menu/tooltip shadows), and implements what we recently changed the default to. Small is the old default value (16px window shadows, 12px menu/tooltip shadows) for people who preferred that.

I had a massive amount of help from @hpereiradacosta; in fact, probably 75% of these changes are his. It's a shame multiple authorship isn't possible. Hugo, feel free to commandeer the revision if you'd like the credit!

Test Plan:
Tested in KDE Neon. New smaller menu shadows for the default window shadow size:

{F5615780}

New shadow chooser UI, with different options:

None (1px border for windows, menus, and tooltips):
{F5623069}

Small (16px window shadows, 12px menu/tooltip shadows); replicates the the old default value:
{F5623071}

Medium (32px window shadows, 14px menu/tooltip shadows):
{F5623072}

Large (64px window shadows, 16px menu/tooltip shadows); the new default:
{F5623073}

Very Large (96px window shadows, 24px menu/tooltip shadows):
{F5623074}

Upgrade story: since this changes the way shadow size is stored in the breezerc file, users who previously had manually set their shadow size to some arbitrary pixel value will now get the new default Large 64 px shadow size.

Reviewers: #vdg, #breeze, hpereiradacosta, abetts, rkflx

Reviewed By: #vdg, hpereiradacosta, abetts, rkflx

Subscribers: rkflx, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D9627