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

[ychin]

Expose scroll button settings and fix misc scroll issues

Allows the user to set whether to show the tabs scroll buttons or not in the preference pane as this is likely going to be a relatively popular configuration.

Also, add new macaction's for scrolling to selected tab and scrolling backward/forward so the user doesn't have to use the mouse to scroll.

Fix misc issues with scrolling to a tab. The existing implementation was animating both the bounds size and position which would cause issues if the user resizes the window while still scrolling. Make sure the animation only touches the position.

Also, the scrolling logic were using the physical locations of the scroll bounds to determine scroll condition, which doesn't account for the fact that it could be in the middle of animation. This led to rapid clicking of the scroll buttons having some clicks being ignored. Instead, fix it to always use the animated destination.

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Use cached images for hover buttons, fix closure memory leaks

MMHoverButton was a bit inefficient in its image management. It always made a new template image which then had to be converted into two other images (image and alternateImage) for every button. Cache at least the template images with weak references so we don't have to keep generating new ones. For now, allow setImage: to create new images but they could be changed to also just use cached images as well.

Also, fix memory leaks in the tabs codebase due to improper closure usage in blocks. They were subtly capturing the self pointer which led to the tab line and hover buttons never getting destroyed. Fix to make sure we never accidentally capture self and try to capture as little as possible.

Another leak happens in the usage of the local event monitor that we use to intercept scroll wheel events. The API contract mandates that we remove the monitor which the code never does. Make sure we do that, and fix up the logic of the event interceptor to be more resilient and works better with third-party software (which could inject horizontal scroll events without holding down the Shift key).

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Clear all tabs while showing / hiding tab line

When the GUI tab line is hidden (either due to set showtabline or set go-=e), Vim stops sending update messages to the GUI. This means the tabs were stale and would never get cleared, and the moment we show the tabs we see a confusing animation when the tabs quicly try to rearrange to match Vim's state. To fix this, just make sure to clean up and remove all of them when we hide the tab line.

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Add ability to turn off animation

This is not exposed in the user preference pane, as it should be a niche case, but it's useful to have a way to turn off animations for either accessibility or user preferences reasons.

[ychin]

See #1120 for original MMTabline implementation.