[Improvement] Menu checkmarks are misleading

[zbee]

The menu for Excel/Filesystem/Settings uses checkmarks to show that a window of that type is open, however this makes me think of checkboxes: i.e. something that can be on or off.

But continuing to click the Excel button does open more Excel or Filesystem windows, so I find the checkmarks to be a misleading indicator ... Except for the Settings window, where it does actually work as being open or not open, it does not allow for multiple instances of itself.

Current behavior:

1. A checkmark, seemingly trying to indicate that the 1 allowable window of that type is already open.
2. Excel and Filesystem allow you to open many windows, but Settings allows only 1, but they use the same indicator.

Expected behavior:

1. A dot to indicate that a window of that type is already open, or no such indicator at all.
2. I would expect Excel and Filesystem to at least have a different indicator from Settings, when they behave differently.

[NotNite]

This isn't super feasible to change the indicator because of how ImGui's MenuItem works, but maybe I can look into it.