updated the instance list icons

[k-vijay-05]

Fixes #6755

Before : (Hard to distinguish which are the locked or unlocked instances on the instance list)

After this PR : (easy to distinguish which are the locked or unlocked instances on the instance list)

[k-vijay-05]

@4ian please review the PR!!

[4ian]

Please ask nicely (no need for these exclamation marks) and wait for a few more days before bumping a PR, especially when there is a week-end during this time.

As for the PR:

• The icons are a good idea but they seem too big compared to the text.
• Z order is different than the Z position, so I think the Z order column should not be shown just after X and Y, but after the layer column.
• We might take the opportunity to replace X and Y by "position" and display them as "X;Y" or "X;Y;Z" instead, that would give more space for other columns?
• The "not locked" icons look a disabled button, so it would be inconsistent with the rest of the UI (a disabled icon can't be clicked)... I think we should brainstorm other ways to do it.

[LuniMoon]

This is an implementation from #6755 (comment) and I didn't specify spacing nor color. Sorry for that
I've attached a pdf with all the specs (size, colour) made with an automation tool (this is why it might look "over specified"). Feel free to use it as a reference if needed:
Specs-Instance panel.pdf

This is the mock up version in the pdf file for the PR:

• The icon sizes are now specified (16px). They cannot be smaller (the tool doesn't have smaller icons).
• Z order has been moved to the right of the layer column.
• I did not merge X and Y positions because the instance panel displays them as separate values, so I prefer to leave them like this for now (I don't know if beginners will understand this inconsistency).
• The "not locked" icon now has the "secondary text" color style instead of the "disable text" color style.

This is the updated version: