detect multiple touch inputs

[Doublonmousse]

Beware : quite experimental and not fully tested.

In particular, the current version creates a lot of

(rnote.exe:11404): GLib-GObject-CRITICAL **: 23:44:59.380: g_boxed_copy: assertion 'src_boxed != NULL' failed

(rnote.exe:11404): GLib-GObject-CRITICAL **: 23:44:59.380: g_boxed_free: assertion 'boxed != NULL' failed

And seems to increase drastically the compilation time.

This replaces event.device().unwrap().num_touches() by another approach, using the fact that each touch sequence is associated to a unique (and constant) sequence. Hence we can use event.event_sequence().as_ptr() as usize to distinguish different touches and follow it.

This prevents any zip-zap when there are multiple fingers on the screen when Draw with Touch is enabled.

Apparently, the EventSequence type is touch specific (so maybe it would be possible to add this into #672 ? Though I don't know if you'd segfault if the function is called on a non touch event).