Port QML Rectangle cropper to QWidget + QPainter

Summary:

1. Port QML canvas based screenshot cropper to QWidget + QPainter based cropper
2. Fix minor bugs with cropper while porting, like system font not being set for size indicator
3. "Include mouse pointer" setting now blends the cursor into the screenshot right away, letting the user see it while cropping, rather than blending it in after cropping

Before:

BUG: 394503
BUG: 374009

Test Plan: Compare with original cropper and check if I missed something

Reviewers: Spectacle, broulik, ngraham, alexeymin

Reviewed By: Spectacle, broulik, ngraham, alexeymin

Subscribers: alexeymin, kpiwowarski, broulik, sharvey, guotao, ngraham, rkflx, Spectacle

Tags: Spectacle

Differential Revision: https://phabricator.kde.org/D12626