[FEATURE] Add on-canvas preview of current color

1) Now when activating color picker in the brush tool
   with Ctrl or any other shortcut, currently selected color
   will be displayed near the cursor on screen.

2) If you start actual color-picking, the two colors will be
   compared side-by-side.

BUG:338128
Fixes task T206
CC:kimageshop@kde.org