Parametric smoothed bracket kit

The kit contains:
- flat bracket - connects several surfaces together;
- L-bracket - connects two surfaces at a given angle;
- corner bracket - connects three surfaces at a 90° angle (horizontal angle is adjustable).

Bracket parameters that are accessible through OpenSCAD Customizer include: bracket angle, thickness, fillet radius, smoothing radius, screw hole padding, hole shape, hole row & column positions, etc. Some of these parameters are "global" and affect the whole bracket, others are "bound" to specified bracket face (or arm), so faces could have different screw holes and different hole layouts.

Also there are "y..." and "z..." parameters that are set to "-1" by default, it means that .scad script will use value from corresponding "x..." parameter instead of "-1". For example, changing "x_screw_thread_diameter" to "5" will also change "y_screw_thread_diameter" and "z_screw_thread_diameter" to "5" if they are set to "-1" in Customizer, but if "z_screw_thread_diameter" is set t