Fix mini editor problem: brush preview wasn't being shown
Fix mini editor animation playback when we're drawing (inside a ToolLoop)

We've to show the document's extra cel only if we are in the current layer
and frame.
Add SetLoopSection command and F2 keyboard shortcut (close #491)
Add KeyAction::Left/RightMouseButton (it's quite experimental at the moment)
Remove MinGW from INSTALL.md
Update cmake command line options for the supported VS versions