Improve category edition UI

Hi,

I added some categories to my Kresus instance and got a bit confused by the UI. I could not find any related issue, so I open this one to discuss it.

First, it took me two times to actually a new category. Indeed, I clicked the "+" button and edited the category name. There was no submit button, so I assumed the new category will be automatically saved upon the input field loosing the focus. Apparently, it is not the case and I have to hit "Enter" explicitly. In my opinion, adding a submit button or autosaving on focus loosing would improve UX.

Then, concerning the auto-suggestion of color, I did not dig yet in the code, but it seems it is always offering similar colors, which are not really distinguishable. When clicking on the color, a full-featured color picker opens, which can be quite impressive to non-tech users. I would tend to think the Gitlab tags approach is a really nice user experience, by offering first a palette of complementary colors, and letting the user customize the color further with a full featured color picker, if they want to.

Concerning the colors themselves, I know Tableau10 color palette which is a set of 10 colors which go well together. This set has been especially designed with accessibility in mind (taking into accounts colorblindness etc).