Add "pseudo-unit-test"

[Findus23]

This would just be a simple bash script that would check the contributing "rules" (that I still need to write if I have some time). e.g:

• do all manually collected files have a file mentioning the source (similar to https://github.com/piwik/piwik-icons/blob/master/src/os/AIX.png.source)
• are any src images smaller than the target size
• does every folder have a "unknown" icon (empty png)

[Findus23]

Added test in a859f7e728b554f2e56bd5861611eea7733235b0, 99d765ac2c769b4040b28052dc0acc16dca66abd, b54179a4ee6b3ac83d2405f658fba23fa555b194 and 912e4d4b7bcddf96c680dbcccd6914fe970f0657