Add config for grayscale icons

[cezarsa]

Hi, I'm not a big fan of colored icons as I think they draw too much attention, however, I do like having different symbols for each file type.

Inspired by the awesome hack used to change the icons opacity I added a new command and configuration to allow switching all icons to grayscale. This is done by adding an SVG filter to each icon file changing its saturation to 0.

Here is an example of how it looks: (I loved it!)

[PKief]

Hey @cezarsa, this is one of the best PRs I've ever received 😄
It has such a good quality that it will be definitely merged 👍

I also like the idea of toggling the greyscale filter on and off. It would be interesting for me if you could give me some feedback if we should make it "true/false" as in this PR or provide an option (same as for the opacity) to customize the saturation by value:

Saturation 1	Saturation 0.5	Saturation 0.3	Saturation 0

Do you think it makes sense to customize the saturation?

[mallowigi]

This feature looks great! I think I’ll implement it as well :)

[cezarsa]

@PKief Thank you. :) I considered exposing the saturation directly but in the end I thought having only a grayscale config would be more straightforward for end users.

However, I do like the flexibility of tweaking with different saturations. I think a compromise may be reached by having a single configuration option configuration.saturation as a float and 2 different commands:

command.grayscale would still exist and set the saturation config to 0 or 1 by enabling or disabling it;
command.saturation would read a float value similar to the opacity command and set the saturation directly.
What do you think about this approach? I can update this PR with this changes.

[PKief]

@cezarsa Yes, this sounds really good 👍
I also think it's better to have the saturation as float value in the settings and do the rest via commands.
It would be great if you could change the PR to this. If you might need any help, don't hesitate to ask! 😃

[cezarsa]

It's done, please take a look.

[PKief]

Thank you very much, it looks very good and has a very high quality.