Improvements and fixes for v3.0 release

[dpizetta]

@juanis2112 please add screenshots with all screens of qdarkstyle example, so I can compare 'ios' version with linux/windows to properly apply the fixes for X in dock and tabs, and undock button in docks. Tks

Writing the main points of our meeting/issues that I'll implement:

• Disconnect tree/list selection/hover color with editor selection color, also check the colors;
• Match/connect pressed menu color with hover/pressed color in the menu (submenus), also check the colors;
• Match Undock/Close buttons in docks and tabs (OS-dependent) Match size/behavior of close/float buttons in docks and tabs #267;
• Hover color for indicators (checkbox/radio) in menus (check on Spyder hamburger menu) may be related to the previous item;
• Fix padding for menus with indicators/icons (Qt version dependent) Enhance spacing in menu with/without icons/indicators - overlaps #214;
• Update main.scss to make the proper descriptions of the new colors;
• Check/update docs based on the new palette system;
• Check over Windows/Linux/Mac and Qt versions 5.12, 5.13, 5.14, 5.15 ( menus with indicators/combo boxes/close-float buttons);

Originally posted by @dpizetta in #262 (comment)

[juanis2112]

Screenshots for Mac:

[dpizetta]

@ccordoba12 @juanis2112 there is a pre-release version in PyPI (pip install QDarkStyle==3.0rc0). I'm going to work on the last details and issues related to Qt versions. Tks

[ccordoba12]

Thanks a lot for your help @dpizetta! We're testing it right now (see spyder-ide/spyder#15048), but things seem to be working fine locally.

We'll have more news for you tomorrow when @juanis2112 adds the light theme to Spyder.