fixes bug with closeTab when the tab didn't exist in the waveWindow cache. also adds Cmd-Shift-W to close a tab (doesn't work for pinned tabs). and restores Cmd-W for killing blocks on pinned tabs
This functions very similarly to VSCode's pinned tab feature. To pin a
tab, you can right-click on it and select "Pin tab" from the context
menu. Once pinned, a tab will be fixed to the left-most edge of the tab
bar, in order of pinning. Pinned tabs can be dragged around like any
others. If you drag an unpinned tab into the pinned tabs section (any
index less than the highest-index pinned tab), it will be pinned. If you
drag a pinned tab out of the pinned tab section, it will be unpinned.
Pinned tabs' close button is replaced with a persistent pin button,
which can be clicked to unpin them. This adds an extra barrier to
accidentally closing a pinned tab. They can still be closed from the
context menu.
