Add option to not give layer a hotkey

[StoneLabs]

Currently every layer has a hotkey assigned based on its position in the layers list (F1-F...). However i find that its quite common to have top level layers that are used infrequently.

Therefore, I propose adding an option to the Layers settings specifying wether a hotkey should be assigned. If the setting is turned off, no hotkey is assigned for this layer / it is skipped.

Additionally this setting would be usefull to toggle for AutoLayers since they need to be used directly only very infrequently (in fact i can only think of single-layer view as the single reason you might wanna select them).