added combobox with example; added FocusUpdate event

Conflicts:
	chili/controls/screen.lua
	chili/core.lua
updated combobox to be its' own component and have it render a triangle to distinguish from the button (add a color/texture configuration?)

Conflicts:
	chili/skins/default/skin.lua
updated combobox to use 2 space identing, renamed method from ComboBox:Close to ComboBox:_CloseWindow
made editbox cursor blink more often and reset blinking state when interacted (it will now show a full line when editbox is being interacted with)
use Spring.GetTimer instead of os.clock
it seems for me os.clock runs double as fast as it should
minor tweaks of LayoutPanel
don't auto call :Invalidate() after :FocusUpdate()
also fix a deadlink issue in KeyPress
2 changes to skins:
* new clone tag: RedButton = Button:Inherit{classname="redbutton"};  ->  skin.redbutton = {clone = "button", backgroundColor = {1,0,0,1}}
* if a skin doesn't contain information for a class, check if it contains them for the inherited class
improve combobox
improve Robocracy button skin
improve Robocracy treeview skin
make chili a bit safer against crashes
skinutils.lua improvements