Add modal stacking and keybinding for modal close

[chisaipete]

I've grabbed the content from #178 and added it as a panel in a new client options modal which also has tabs for color settings and language selection. This also includes the code to do z-index stacking of the modals in the client, also a new keybinding for Escape to close all open modals. This should partially solve #130, as it gives a home for keybindings to be set.