Update version and copyright year

Version 0.15

Add missing changelog.
v0.15 tag/release was previously set on cc5e1da about 3 weeks ago, will attempt to change tag into this commit.

Merge branch 'master' of https://github.com/epezent/implot

Merge branch 'master' of https://github.com/epezent/implot

buffer overflow fix

fix imgui backward compat, YAxis[3], and add new demo benchmark optio…

…n for LineG

Subplots (#203)

* SubPlots: Y axis padding over multiple plots

* Align Plots Group

new signature ID, ImPool  to store padding data, demo

* Align plots orientation (vertically, horizontally)

vertical will align Y axis, horizontal will align X axis.
*signature changed

* ImPlotOrientation used as flag for 2D grids of aligned plots

#144 (comment)

* AlignPlots updates to merge with v.0.9

* Sync to v0.9 20210127

* subplots proto

* make link flags work

* stuff

* add multi-line centered titles

* subplots work

* flag ideas

* better subplot positioning

* resizable subplots

* subplot shared items

* subplot ratios

* some cleanup and refactor

* some cleanup and refactor

* refactors and demo reorganization

* context menus...almost done!

* context menus, bug fixes

* active id

* make implot use ButtonBehavior throughout

* bug fixes

* more bug fixes

* tweaks

* fix id issue

* finish work on subplots

Co-authored-by: ozlb <[email protected]>

Update README.md

update tables demo to use TableHeadersRow

trim white space