some undo issue preventions (clearImage & deleteSelection)

1. Prevent from unnecessary updates
2. Prevent from extra undo backups
3. Optimised if-else
Add keyframe struct
Add keyframe class
Add camera class in its own file
Add keyframe helper methods to Layer.cpp

I think we should move all the keyframe stuff to layer to avoid casting
Add scrubNextKeyframe and scrubPreviousKeyframe methods
Fix interesting bug in scribblearea when pressing keys?

couldn't really reproduce consistently
Add previous and nextkeyframe actions to the menu
Merge pull request #108 from eVoLuTiVeMEDIA/master

Some undo issue preventions for clearImage & deleteSelection
Merge pull request #110 from wesen/feature/navigate-drawings

Navigate to previous and next keyframes with < and >
Resolve conflict
Add recentfile menu to keep track of recent files
ask for color name after creating a new color
Return layers when creating new ones in Object
Ask for layer properties name when creating a new layer.

This really needs to be refactored. Maybe something like a LayerManager that is a child of Object / Editor. There is too much going on in these two classes.
Update the timeline when renaming a layer
update max frame before scrubbing forward, in case a new keyframe was added
disable coalescing only in actual drawing stroke tools
forgot to endstroke in erasertool
Fix jitteriness of movetool by fixing the reference point