Move translate_keysyms to bindings.[ch]

Additionally add a check so the function only handles bindings of type
B_KEYBOARD to prepare for the new bindmouse feature.
Bugfix: don't try to resize docks by command

cmd_resize does not implement a method to resize dock clients.

A command like `[instance=".*"] resize grow width 160 px or 16 ppt`
when a dock client matched would crash i3.

fixes #1201
Merge branch 'fix-resize'
Merge branch 'master' into next