Detect mouse-triggered selection in order to switch its visual selection to Selection mode

[tombh]

With commit e3f8943 there was an improvement to selection behaviour in order to support snippet plugins that auto-select the parts of snippet that a user needs to fill. However, the downside to the approach I took is that now mouse selection selects in Visual mode, which is not like normal selection behaviour, namely pressing any key doesn't exit the selection and indeed SHIFT+arrow does unexpected things (if you're not used to Vim).

So I need to think of a way to fix that.