Add command line option --export-png-color-mode for setting bit-depth and color-type

A command line option is added (--export-png-color-mode) which resembles the export dialog's "Bit depth" setting. (Gray_1, Gray_2, ..., RGB_8, ..., RGBA_8, etc.)

Note that the dialog says "Bit depth", but internally this splits into "bit depth" and "color type". I chose the name "color-mode" to stay away from both parameters, not sure, whether there is a more appropriate name.

Similar to the code in dialog/export.cpp I map the array of valid options for color-mode to values for bit-depth and color-type. The actual arrays are copy&pasted from there.

The code style pipeline complains. (0,0,0 -> 0, 0, 0 and also the placement of "void export_png_color_mode" on different lines)

Should "void export_png_color" (in actions-output.cpp) be styled as the similar functions above it (so void on one line and function name on next) or according to the suggested corrections from the style check (same line)?