Webclient: canvas event handlers are lost when switching back from geomap

[jri]

Once a geomap was opened and then switched back to a normal topicmap the canvas event listeners are lost. As a consequence:

• Moving the topicmap doesn't work anymore
• Moving topic/assoc clusters doesn't work anymore
• The topicmap context menu and assoc context menus don't work anymore

This bug was introduced in DM 4.7 in the course of refactoring the Canvas element (in conjunction with touch devices support).

[Jörg Richter <jri@…>]

In 8fbe3155363d1e180720bbb29b44e691c8bfc931/deepamehta:

Webclient fix: keep canvas event handlers (#896).

When switching to a geomap the canvas element keeps its event handlers.
This is done by using jQuery event delegation.

Canvas event handling was broken in DM 4.7

See #896.