Correct column widths

[lukewalczak]

Describe the bug
After using UnitControls in `Columns introduced in the PR there is a regression in distributing columns width.

Basically some functions such as hasExplicitColumnWidths or toWidthPrecision from columns/utils.js cannot work since there is a check Number.isFinite(value). Value always comes with the unit, so the result is false. Unfortunately it has an effect on columns width distribution (please notice that on presented gif, the third column width is different)

before PR	after PR

Function called getTotalColumnsWidth used within getRedistributedColumnWidths sums the width attribute of the individual column defined in % (correct me if I'm wrong). With the approach where width can be defined in different units, function mentioned above should take into account somehow.

To reproduce
Steps to reproduce the behavior:

1. Open a post
2. Add columns block
3. Choose variation 30/70
4. Add third column
5. Observe width is not distributed properly

Expected behavior

Width should be distributed properly as before introducing the UnitControls

Screenshots

Added above

Editor version (please complete the following information):

• Gutenberg 9.2

Desktop (please complete the following information):

• OS: macOS
• Browser: Chrome
• Version: 86.0.4240.75