Update initialize management

[skoudoro]

this PR:

• remove initialize from ShowManager init. : it creates a unwanted window (render is call inside initialize)
• add initialize in add_timer_callback . iren need to be initialized before adding an observer

[Garyfallidis]

A clarification here. The initializer is needed only with timer_callbacks but not in multithreaded cases.

[skoudoro]

A clarification here. The initializer is needed only with timer_callbacks but not in multithreaded cases.

I confirm

[Garyfallidis]

Merging. This closes #709 too.