Timeline: Add button and handles to configure onionskin range
Timeline: Esc key deselect the selected range of frames/layers
Add ability to change the properties of several frames (a range of frames) at once
Fix some compilations problems in MinGW

Anyway without DirectX header files (ddraw.h specifically) we cannot
compile Allegro source code with MinGW.
StandbyState::onUpdateStatusBar(): show frame duration
Timeline: add status bar text when we hover different elements
Timeline: show frame/layer/cel properties with double-click
Timeline: fix problem setting the duration of a frame range
Avoid negative values for Timeline::m_separator_x (so the user can expand it again)
Timeline: Add support to delete several cels at once