Add a CompoundEvent class which represents
an event that can be represented as a logical
composition of several events working together.

As a first usage of this class, the 'longpress' event
is introduced.
This event fires once when a finger is placed against the
screen for 400 milliseconds without moving or lifting.

Select a word when long-pressing on it.
handles
Make selections touch-modifiable with handles.

Two new compound events:
* 'drag', which depends on 'touchstart', 'touchmove',
and 'touchend. It fires whenever a finger is placed
on an element with class 'draggable', and then moved.
* 'dragstop', which depends on 'drag' and 'touchend'.
It fires whenever the finger is lifted after a drag
on a draggable element.

SessionController, upon encountering a drag event on
a selection handle, updates the shadow cursor's selection
to match the one prescribed by the handle movement, thus
being very fast.

When a dragstop event is encountered, the selection of
the shadow cursor is fed to the local member's cursor
via SelectionController, thus utilizing an op.
Hide selection handles of other users.