Add magnifier for rectangle mode

Summary:
Add magnifier for rectangle mode, it can show 5x of the mouse pointer

Depends on D11598

FEATURE: 392482

Test Plan:
In rectangle mode:

1. select rectangle, check magnifier
2. adjust selection, check magnifier
3. change the selection to any size and check magnifier
4. test magnifier on multi-screen
5. test magnifier in HiDPI mode

Reviewers: Spectacle, rkflx, ngraham, alexeymin

Reviewed By: Spectacle, rkflx, ngraham, alexeymin

Subscribers: alexeymin, rkflx, ngraham

Differential Revision: https://phabricator.kde.org/D11599