Tabs: Expose option to show scroll tab buttons, fix tab scroll behavior, fix memory leaks, cache images and other cleanups#1536

ychin merged 4 commits intomacvim-dev:mastermacvim-dev/macvim:masterfrom

ychin:mmtabs-add-scrollbutton-prefs-other-fixupsychin/macvim:mmtabs-add-scrollbutton-prefs-other-fixupsCopy head branch name to clipboard