Implement a full UI for the Bookmarks feature

PR #8481 implements a bookmarks feature for Spyder’s Editor. Here, we will discuss and decide on an appropriate UI for the same.

@ccordoba12 's relevant comments:

For the UI part I’d like to see:

1. An icon next to the line that has a bookmark.
2. A panel to browse bookmarks.
3. A keyboard shortcut to move to the next/previous bookmark.

We can add the list of bookmarks to the file switcher or add a shortcut to move to the bookmarks pane and filter file names with the keyboard there.

@CAM-Gerlach 's proposed UI:

• Highlighted line number (to save space) of lines with bookmarks
• Bookmarks Manager, based on either Breakpoint panel (has all needed management features) or Outline Explorer (file-based)
• Previous/next shortcut based on most recently accessed order (or maybe even integrate it with Ctrl-Tab?)
• Ability to assign (either preset, or user-defined) shortcuts to specific bookmarks via the Manager for quick access
• Maybe a Show bookmarks option in the Outline Explorer?

[agentmorris]

Weighing in here with UI opinions, apologies for some duplication of a comment I made on 8481 (before I knew this PR was the right place to discuss the UI).

My opinion should be given very limited weight here, because I don’t have the familiarity w/Spyder to implement anything. Nonetheless, weighing in with a vote for a UI more or less identical to the very simple approach that both Matlab and Visual Studio take, namely:

• Ctrl-F2 sets or removes a bookmark on the current line (there are no character-level bookmarks). This is indicated with a dot in the line numbering area, where breakpoints are indicated.
• F2 cycles the cursor position through bookmarks in all open files. (Shift-F2 cycles backwards).

These are Matlab shortcuts; the keyboard shortcuts are different in VS, but the paradigm is identical.

There is no numbering to bookmarks. AFAIK bookmarks are not saved in either Matlab or VS.

I personally prefer this simple approach to vi-style named/numbered bookmarks; the way I use this (and apparently the way most Matlab users use this, else they presumably would have changed it) is pretty temporary (but important!), and the cognitive load of remembering multiple bookmark numbers would be too much.

This does not serve the purpose of true persistent favorites that some comments have expressed a preference for above.

Excited to see any form of this feature happening!

[goanpeca]

Yes, I agree with this.