Add "slack" width constraint modes

[ahmacleod]

Adds two width constraint modes: eq-container-slack, gte-container-slack. These build on their non-"slack" counterparts by allocating excess container whitespace into an additional column that vanishes when it is no longer needed. This resembles behavior in applications like Excel and Google sheets.

BONUS ADDITION: initialFillMode parameter that is used exclusively in slack modes to perform the initial column layout. Values and default are the same as fillMode. In eq-container-slack mode, fillMode and initialFillMode can be set to different values. The former is used for enforcing the width constraint, while the latter is used only for initial sizing. See docs & tests for examples.

Which constraint modes use which fill modes:

Advantages:

• More closely resembles resize behavior in applications like Excel, Google Sheets
• Frees the rightmost column; e.g. if table has two columns and the right column is right-aligned, this makes it possible to bring the right column values to the left for easier comparison.
• Preserves the width of the rightmost column

Notes:

• The slack column cannot be reordered or manually resized
• There is no slack counterpart for lte-container or none
• fillMode has no effect in gte-container-slack mode, but does still apply when enlarging columns beyond the container size in eq-container-slack mode
• resizeMode can be fluid, but standard (shown below) provides a more consistent UX
• initialFillMode should be used with slack modes if a "responsive" initial layout is desired

eq-container-slack

gte-container-slack

[twokul]

Noted a few things. LGTM otherwise! 🎉

[ahmacleod]

Refactored to get rid of isInitialRun entirely. We now have performInitialLayout() and ensureWidthConstraint(). No funny business.

[twokul]

The code has changed since my last review. Noted a few things ✍️