MATE desktop fixes (updated)

Last week I upgraded my Linux Mint 18 MATE desktop distro to 18.3. With the massive progresses GNU/Linux has done in the desktop, those kind of upgrades use to be simple tasks and no more hassle is to be expected. Except this time where I had several a GUI-related annoyances. 1. MATE panel transparency I recently became addicted to /r/unixporn and made myself a shiny modern desktop made of MATE and rofi.