enhance visibility and layout of buttons

[erwindon]

buttons have poor layout: the spacing is ad-hoc
buttons have poor visibility: the button image is usually a visual character so that the active area is unclear

This PR creates a grey background colour for each button.
This immediately makes the layout problem more visible, so the layout instructions are re-worked.

[erwindon]

@dawidmalina
In late 2018 and early 2019, you provided an updated design for SaltGUI.
This design has been still remains, but there were a few minor issues with the (menu) buttons.

Because, over time, a few more buttons were added to several screens:

• button for panel-level menu (until then: only a menu item existed per table row)
• button to start search in tables
• button to pause/continue automatic screen updates
• and a few more

And the buttons were not always recognizable since they were typically constructed using a single graphical character. So it was not always clear that the character even represented a button or a popup-menu. Also the extra margins around these characters were not visible. This resulted in an odd look for buttons that were near the edge of a block.

So a bit improvement was needed...

This PR cleans up the graphical design wrt to regular buttons and popup-menu buttons.

Can you please have a look at the graphical side of the result?
If/when you feel like it of course, as you may have moved away from SaltGUI or even away from SaltStack.

thx!

[dawidmalina]

Hi @erwindon I can take a look on this but not within next 2 weeks. Never the less I must agreed with you - my involment with project, saltstack is close to zero right now as I am focusing now on different topics. I know that SaltGUI is in good hands :)

[erwindon]

@dawidmalina thx for the quick response and the compliment! I'll now not let this PR depend on this, but feel free to still take a look at it at any time and send any remarks this way.