They are already present in the program
Line: select pencil but hold shift when drawing and it will draw a line
Mirror: in the properties of the line tool are options to mirror in the x and y axis (the mirror lines can be dragged)
Shapes: circle shape can be selected from the brush type of pencil along with its size. also, you can also add custom shapes (as brushes) by exporting a shape (e.g a square shape) as .png and import it back in pixelorama as a brush if you want a brush to be permanent simply move the .png to Pixelorama/pixelorama_data/Brushes/
you can also use this feature for tile set painting