Retina support for database icons

[gyli]

Summary

The current data icons are quite rough in retina screens.
https://cl.ly/039617

Desired Behavior

Use icons with higher resolution.

Possible Solution

Looks like the icons are from Nuvola icon set created by David Vignoni.
A full list of icons might be found in the following places:
https://commons.wikimedia.org/wiki/Category:Nuvola_icons
https://www.iconfinder.com/iconsets/nuvola2

Or it could be replaced with a new, non-Nuvola icon set.
Here are some examples using different icon sets:
Keepassium https://keepassium.com/
MacPass https://github.com/MacPass/MacPass/tree/master/Assets/Icons%20as%20PDF%20files

[droidmonkey]

Those icons are ancient, and in my opinion, need to be completely replaced. They are actually from the original KeePass.

[phoerious]

It's just a bunch of tiny Nuvola icons. Ugly as ever.

[gyli]

I would also like to mention that, the icon of the toolbar is also fuzzy on Mac's retina screen. and I doubt if it's well scaled.
https://cl.ly/651458

[phoerious]

Closing this in favour of #475.