Feature: Add Increase/Decrease Font Size Keybindings

Often I am screensharing and want to temporarily change font sizes, having a command and keybinding for that would be very helpful.

I opened this issue to hear suggestions on the best set of keybindings for these two commands.

The way it's going to work is that the font size changed will be persisted only until the Oni session ends, it should not affect the user configuration file.

After suggestions, I am willing to implement this, thanks!