New function `configuration-layer/layer-declaredp`
Move window-numbering package config into its own init function
Neotree window number is now always 0

This other windows keep their number
Add a feature to toggle transparency

This sets the initial `alpha` frame parameter so that transparency could
be toggled later on. The level of opacity can be configured from a
user's .spacemacs through the variables `spacemacs-active-transparency`
and `spacemacs-inactive-transparency`.