Is your feature request related to a problem? Please describe.
When I place a schematic into the world, there is no easy or convenient way to change all blocks of a specific type. This is especially true for redstone-related builds where only block types (solid, slab block, transparent etc.) matter.

Describe the solution you'd like
When creating a schematic, there could be a way to set a specific block as 'Generic' type. (for example)
When this schematic is loaded in, any other solid block can be used in place of the solid block that was a placeholder.
This could also work for other types of blocks, i.e. Solid, Transparent, Half-block etc. and multiple modes could be selected for a block type.
If this explanation is too messy, I could try and explain in another way.

Describe alternatives you've considered
I am aware of the schematicRebuildReplaceAll feature, but there is no GUI method to do it.

Additional notes

1. I am also aware that #230 exists and is open. It describes a GUI-related way to solve this problem. This is a related feature request that also helps solve other small problems, so I have created a new feature request.
2. #313 describes a solution, but this may be easier to implement as compared to that. Both features should be possible to implement without conflict (I believe)