fix #5659, Unify the icon color settings for Menu and Toolbar, and fixed some strange places

[BroKun]

What it does

fix #5659

• Use the opacity property to display the availability of the menu, which is better for colored icons.
• Use the same opacity value when disabled.
• Make sure the bottom tabbar icon is vertically aligned with the text.
• Improved UI of search-in-workspace
  • Use pointer cursor when replace-all button is available.
  • Search-detail and search-header are horizontally aligned.
  • Use absolute position to make the clickable area of the toggle button larger.

How to test

1. The menu bar should look the same.
2. Use a colored icon in the menu bar, and the icon color should change if the menu is disabled.
3. The icons in the bottom tab-bar should always be vertically aligned with the text.
4. search-in-workspace
   1. The disabled icons should look the same grayscale.
   2. Input box are horizontally left aligned.
   3. The show-detail button is always clickable.
   4. When the replace-all button is enable, the cursor should be pointer.

Review checklist

• as an author, I have thoroughly tested my changes and carefully followed the review guidelines
• I have thoroughly tested my changes in Chrome 76.0.3809.132 and FireFox 69.0 on macOS 10.14.5.

Reminder for reviewers

• as a reviewer, I agree to behave in accordance with the review guidelines

[jbicker]

These are very good improvements. Thank you!
But I agree with @akosyakov that we should have a css variable for the 'disable' opacity as he mentioned here.

[akosyakov]

code changes looks good to me, please rely on @jbicker's approve for testing

[jbicker]

Looks good to me too!

[akosyakov]

@BroKun please merge