Fix shader compilation for some node names with special characters

[MoritzBrueckner]

This PR fixes shader compilation for nodes or sockets with § or $ in their names and also fixes more cases where the shader compiler would output a warning because of consecutive underscores.

ERROR: [...]\compiled\Shaders\awning01_red__props_blend_mesh.frag.glsl:9: '$' : unexpected token

WARNING: [...]\compiled\Shaders\chimney005__props_blend_mesh.frag.glsl:38: 'VertexLitGeneric_GroupInput__bumpmap_texture__res' : identifiers containing consecutive underscores ("__") are reserved
WARNING: [...]\compiled\Shaders\chimney005__props_blend_mesh.frag.glsl:67: 'VertexLitGeneric_GroupInput__selfillummask_texturealpha__res' : identifiers containing consecutive underscores ("__") are reserved

There are more situations where Armory generates shader code with double underscores, but for some reason I couldn't reproduce a warning in the other cases I found... Nonetheless it may be a good idea to implement a function similar to utils.safestr() and utils.safesrc() but for shader identifiers, this way we could remove some of the (potentially redundant) double underscore correction code from other places and fix this whole class of errors once and for all. This function could also be used to ensure that generated variable names are unique (e.g. by appending consecutive numbers), I believe there are some cases where naming conflicts are possible...