Rename app::SelectionSettings to app::UISelectionSettingsImpl

The class UISelectionSettingsImpl is the implementation
of ISelectionSettingsImpl so it doesn't need to be available from
ui_settings_impl.h. Also we can use anonymous namespaces to put classes
that are used only inside ui_settings_impl.cpp.
Remove layers button from StatusBar widget
Timeline: improve look & feel of range selections
Fix bug mixing image buffers in ExpandCelCanvas for new cels

Bug introduced from commit 48864b440b82a710076a8af66d98c95343407e40