FEATURE: Mouse button clicks can now be bound in keymaps by using
names like `"LeftClick"` or `"Ctrl-Alt-MiddleTripleClick"`. When bound
to a function, that function will be passed the position of the click
as second argument.

FEATURE: The behavior of mouse selection and dragging can now be
customized with the
[`configureMouse`](http://codemirror.net/doc/manual.html#option_configureMouse)
option.

Avoid duplicates for #4719

Issue #4697

Closes #4576.

Issue #4537

Issue #4485

Closes #4478

Issue #4417

Issue #4413

Adds an option to keep the active line highlighted even when text
inside the line is selected.

And use it in the merge addon to get the proper offsets for merge buttons

Issue #4364

Issue #4288