[Feature] Transparency when always on top

[PetiPandaRou]

When Always on Top, the display of Pomotroid is taking a lot of display space. It could be a good idea that we can customize the transparency of the display, in order to at least see what is display beyond.

[culmat]

ideally only have the background transparent / show buttons on mouse over see https://www.electronjs.org/docs/api/frameless-window#transparent-window