Support different options for `g:ale_lint_on_enter` for running linters less often

When linters are very expensive (such as `clang-tidy`) it places a penalty on using vim by opening buffers without using windows and switching back and forth between them. With this, ALE will re-lint each buffer when re-entering it. When the buffers are opened as windows side by side or as tabs, this does not occur.

I think that a reasonably practical workaround might be to `let g:ale_lint_on_enter = 0` but my thought was this could be made a tiny bit more intelligent somehow.