Mode line improvements

[danielmartin]

With some standard mode lines, lsp-treemacs always shows "Treemacs" in the mode line, making panes difficult to distinguish.

This PR solves the problem by setting either of these two buffer local variables:

• mode-name. For Doom/Spacemacs/Moody, Treemacs builds a custom mode-line-format that displays the mode name along with other info in a propertized way.
• mode-line-format. In standard mode lines, Treemacs modifies the mode-line-format to show " Treemacs ", so from lsp-treemacs we'll do the same but show a descriptive pane title instead.

I think this PR supersedes https://github.com/emacs-lsp/lsp-treemacs/pull/10, so I've removed the major modes that that PR created.

[danielmartin]

@yyoncho What we discussed in Gitter. I've tested with a standard mode line and a Doom mode line and this approach seems to work as expected. It wouldn't be a bad idea to test this with Spacemacs mode line as well.

[yyoncho]

Looks good! We should do the same for dap-mode as well. I tested with spaceline and it works fine.