Angled Table Headers

[ocornut]

Posting a thread here as it hasn't be discussed on github before afaik.
Linking to #3740

• Added angled headers support. You need to set ImGuiTableColumnFlags_AngledHeader on selected columns and call TableAngledHeadersRow().
• Added style.TableAngledHeadersAngle style option.
• Added ImGuiTableFlags_HighlightHoveredColumn table flag.
• Added Demo->Tables->Angled Headers.

Some constraints/possible improvements:

• This can only be submitted once per table, and as the first row.
• If used, the locked/frozen columns must not contains angled header.
• The angled headers currently highlight but are not interactive (need to click on the empty rectangle below it for reorder, context-menu etc.).
• It's currently using an extra draw call that would probably be removed with a bit of rework. I'll do that eventually.

[ocornut]

Pushed various fixes and support for multi-line labels in angled headers:

[ocornut]

FYI as part of https://github.com/ocornut/imgui/releases/tag/v1.90.6 @thedmd and I added support for adjusting alignments of angled headers, and made fixes for multi-line headers: (it wasn't simple)

Closing this now!