Remove the key "keybinding_behavior_strict" from preview.conf

[ralf3u]

Reason one
The key "keybinding_behavior_strict" was introduced in preview.conf because I opened #67. To that time I thought that the shortcut name "Toggle focus between editor and preview" does not allow to come from the message window – I was concentrated on the word "between". Last week I recognized that I was wrong. The shortcut allows to come from the message window because the shortcut name is not "Toggle focus only between editor and preview". So, there is no need for "keybinding_behavior_strict".

Reason two
All shortcuts should be listed in Preferences in Keybindings, and all shortcuts should work as they are noticed in the Preferences in Keybindings.

Remark
I'm sorry that I opened issue number 67.

Version
Preview 0.2.4+r34.g99df95e