Improve image editor

[milasudril]

Some improvements to image editor

• [x] Scroll wheel: Change brush size
• [x] LMB: paint with current collor
• [x] Shift + LMB: floodfill with current color
• [x] RMB: Paint with pure alpha (should use a larger brush)
• [x] Shift + RMB: floodfill with pure alpha
• [x] "Navigation" buttons: Switch palette
• [x] Shift + "Navigation" buttons: Switch image
• [x] Fx: Select brush
• [x] Outline for image size and center in ImageEditor viewport
• [x] Show brush size in ImageEditor viewport (requires that true brush size calculation is moved)

[milasudril]

Completed as of 2f200a2